The economics of a project like this are enough to have you certified insane.  You’ll need to buy TWO Minicraft 1/144 scale P-51D kits to build one P-82 aircraft model.  Pick up some cyanacrylate (CYA) glue and accelerator spray.  Dr. Frankenstein likes to use Insta-Cure brand medium consistency gap-filling type, and Insta-Set accelerator.  Some sort of modeling putty will be needed, too.  Squadron “Monster” green or Tamiya “Pallid” gray putty are good choices.  While at the hobby shop, ignore anyone who stares at you or laughs at your mad-scientist getup or wild hair.
 
Before you shop for body parts, always make a list of what you need, especially if your castle is way out in the woods and far away from your hobby shop.  I forgot the most important ingredient, sheet styrene plastic.  Dr. Frankenstein likes to have both .040” and .020” thicknesses on hand.  I sent Igor into the village to pick up some sheet plastic, but he brought back the wrong kind.  He said he got it from Abby-somebody.  I think I’ll go into town myself to pick up items from now on.

A P-82 next to its much smaller Mustang parent:  three kits, two models!  Radical conversions such as this are lots of fun! 

Avoid headaches!  Plan carefully before attempting a major conversion to make sure you have everything you may need to see the project through.

For any project like this, you need good plans.  A clear, well detailed three-view drawing is great.  For my project, I used Squadron’s F-82 Mini In Action book.  You might be able to find this book in your hobby shop, or by mail unless the postman refuses to come near your castle like mine does.  Pick the variant you like that has a complete three-view drawing.  You’ll need to reduce (or enlarge) the drawing to the correct scale.  Dr. Frankenstein has a reducing/enlarging flat bed copier/scanner in his laboratory.  If your lab is not as well equipped, most print/copy stores have self-service copy machines that enlarge and reduce by percentage.

First, determine the length that the finished model will actually be.  To do this, divide the length dimension of the prototype you are modeling by your chosen scale.  Hopefully, you learned math and the Metric System at Mad Doctor school.  It's a lot easier to do this sort of work using the Metric system than to mess with fractions of an inch.  For example, the P-82B is 11.91 meters feet long, or 1191 centimeters (cm).  Divide 1191 cm by 144 (the scale we are working with).  The answer is 8.3 cm.  This will be the length of the model when we are done.  Now, measure the drawing.  Divide the length of the model (8.3 cm) by the length of the drawing (9.8 cm).  The answer (85%) is the percentage you will need to shrink the drawing on your copier.  Sometimes the precision of the copier may be a little off; be sure you measure the copy and adjust the percentage either way to get the size of the copied drawing just where you want it.  Also, be aware that some copier’s optics may distort the copy one way or the other as well.  If you are certain the length of your subject is correct on the particular drawing, measure the wingspan just to make sure.  If you scanned the drawing to your computer, stretching it one way or the other to correct an error like this is simple.  We mad scientists are always toying with nature in some way, so this kind of thing is easy for us. 

Slice and dice!  Refer to drawings often while making cuts.  The three views in Squadron’s book were different relative sizes—each had to be measured and reduced separately.
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Fabricate sheet styrene plugs to extend the fuselage.  Several small pieces can be used; trim them to match the fuselage contours.
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Lay the fuselage parts over your drawing and determine where you will need to make cuts.  In this conversion, the P-51 fuselage has to be lengthened and deepened.  Use a fine-point permanent marker (like a Sharpie pen) to draw lines on the parts where you will make the cuts.  A sharp knife or razor saw will make short work of them.  For the double-fuselage P-82, you’ll need to do everything twice.  Crazy!
 
Using the drawing as a reference, splice in pieces of sheet styrene plastic to one half only.  Liquid cement allows things to be adjusted; CYA glue provides strength to the joints.  Make sure the fuselage half lays perfectly flat; use a piece of glass if it helps.  Reinforce the back of the parts with plenty of CYA glue and plastic scraps.  The thin tail was reinforced with a small piece of wire set into an indentation.  While waiting for the glue to dry, I sprayed the castle for spiders.  Mrs. Frankenstein had been complaining about them.

Two finished left halves.  Reinforce with CYA glue, styrene, and wire if necessary.  Ugly-on-the-inside doesn't matter here.

If your finished fuselage half lays perfectly flat, you can cut and splice the pieces from the other half directly onto it.  Check for symmetry constantly as you add parts.  After major gluing sessions, it’s a good idea to let things dry for several hours. While waiting, you can call out for a pizza.  If the pizza delivery folks are nervous about coming to your castle because of previous “unfortunate incidents” with your various creations, ask that they send someone who is new to the area.  This tip also works with milk delivery folks and utility meter readers.

The wing needs its leading edge extended and the wheel well modified.  Sheet styrene and CYA glue are used to do this.  Also, a new wing center section will have to be built from scratch. New lines representing control surfaces will need to be scribed.  Once these assemblies are finished, they should be painted with a light gray primer.  This will show imperfections and blemishes that will need to be corrected later.  After this step, it is recommended to take a break.  Playing Bach’s Toccata and Fugue in D Minor on your organ with a reggae beat for a change will be refreshing and inspiring.
 
Once the paint is dry, you will see where joints may need filling.  Deeper ones should be filled with more CYA glue.  Putty will work well for more shallow indentations.  Thin a dab of it with Testors liquid cement, and rub it into the correct spot with a toothpick.  Let the putty dry for awhile, especially if the area where you live is foggy and dark all the time.  Avoiding anxious neighbors wielding torches and pitchforks may help occupy the time needed for the putty to cure.  This will help prevent sinkholes in the model that might become visible over time.

​The trick to getting a perfect creation is to repeat puttying, wet-sanding, and priming until all undesirable blemishes are gone.  Varying the shade of your gray paint for each coat helps ferret out high and low spots.  Stir a little black paint into white; Dr. Frankenstein actually uses glossy white paint with a few drops of flat black as primer.  After painting, inspect closely for blemishes that still need correcting.  Carefully check the contours against the drawings and reference photographs to see if they are right as well.  If there are high spots, don’t throw a brain-addled fit; simply sand them down.  Likewise, thinned putty can be dabbed into low spots.

Right half sections are glued to the completed left half piece-by-piece.  Everything should end up straight if the left half laid perfectly flat on its centerline!

After wet-sanding, scrub the subassemblies with a toothbrush and soapy water.  Rinse well, and let air-dry to avoid towel lint.  Drying the model with a towel also charges the plastic so it attracts castle dust, so resist the temptation and give it time to air-dry.  While waiting, storm over to the town hall if you need to.  Zoning laws favoring things on your property like the undead, exotic pets, and bats in your belfry need to be defended sometimes.

Unlike the P-51 kit, The P-82 wing has a straight leading edge.  You’ll need to add a piece of plastic to duplicate it.  The wheel well must be modified as well.  CYA glue used as filler contours the added parts seamlessly into the wing

A new wing center section is scratchbuilt from sheet styrene.  Lots of sanding creates the airfoil.  New control surface lines are scribed; check the drawing for locations.  Fuselages have been primed and puttied; they are ready for another round of wet-sanding.

Don’t attempt final assembly until the subassemblies are sufficiently blemish-free.  It is easier to work with them without the other parts in the way.  While this ongoing work progresses, cleaning up and finishing things like props and canopies can be done.  Dipping the canopies in liquid Future floor wax makes them crystal-clear.  Sundries like landing gear doors, cockpit interior parts, intake extensions, and the horizontal tail will all need to be scratchbuilt, or cut and/or carved from sheet plastic pieces.  Landing gear was tricky; I had to cut off the original molded-in gear door leaving the wheel and strut intact.  You just can’t be normal to attempt this kind of work.

Note kit air intakes and scratchbuilt extensions.  Kit tail wheel well parts were glued into notches 

While performing final assembly, check and recheck alignment.  You’ll need to do lots of dry-fitting and trimming to get everything to look right and line up.  Puttying the joints will then be necessary; prime, putty, wet-sand, and repeat as needed until these joints are perfect.  I finished up with a black, glossy enamel coat in preparation for my metal finish.  I used different shades of Alclad metal finish on my model and disturbed the natural order of things by polishing with SNJ’s metal powder.  Use a respirator mask and gloves when applying such products.  Even if you are unstable mentally like all decent mad doctors, you still don’t want to risk hurting yourself physically.

For markings, one of the XP-82 prototypes is an easy option.  Insignias from the kit can be used and as well as a few numbers from a Scalemaster lettering sheet. 
 
​Now, you can apply decals of your choice and finish the model.  If you want, you can wait for a stormy night with lots of thunder and lightning for the best dramatic effect.  Don’t forget to laugh demonically as you apply the finishing touches.  Mu hu hu ha ha ha ha ha!  It’s alive!  It’s alive!

Here’s what the bottom looks like after everything is done.  

Out-of-Box and Out-of-Box Not!  Looks like three planes parked, doesn’t it?

At left is the Amazon Sponsor Link for Minicraft's P-51D in 1/144 scale.