The MPC falcon takes a LOT of grief but with soem dedication it does really build nicely. But reading your original post I'm not sure you realize how loaded a question you're asking. The way I see it there are about 3 levels of MPC falcon modifications... and which way you go determines how crazy you are... an I oughta know I've gone down all of the roads
Level 1 you decide to buy an upgrade kit and build a darn fine reperesentation of the Falcon. FalconKits or Blue moon make replacement parts for the side walls and a few other bits. Using them will produce a very respectable result.
Level 2 - you decide that you want to try your hand at scratch building and want to partially relive the process that ILM when through when they built the falcon. you purchase some sacrificial, 1/72 tank kits, 1700 (or smaller) battleships, assundry helicopters, jets and cars, and have a go at kitbashing a ton of details that "look functional".
while:
- Correcting the height of the sidewalls.
- adding detail to the engine deck area
- the crossmember pieces missing form the kit cockpit cone
Level 3 - you've decided that while the basic geometry is close you just can't live with what's not right with this model you:
- correct the diameter of the cockpit tube and cockpit
- correct the sidewall height
- correct the diameter of the radar dish
- correct the curvature of the hull (various methods but the most effective in my erperience is placing a plug in the center, clamp the outer rings, and GENTLY apply heat to soften the plastic enough to "take" the new shape - long slow but effective.
- research the studio model and get the parts maps form the studio scale guys, then research the same model "subjects" at 1/4 the scale used on the 5 footer or 1/2 the scale used on the 32" (depending on which you're replicating).
- Buy all said models and try to steal the same/similar bits form the smaller subjects to use on the MPC replicating the studio look as closely as possible
- scratchbuilding anything that cannot be found by above utilizing the studio scale reference
- practicing on every scrap piece of styrene or other material to perfect the airbrushing, and weathering techniques to get the "just right" amount of grunge
- Researching and experimenting with every shade of gray, until you finally realize that you're gonna have to learn to mix paint and airbrush if you're ever going to be satisfied with the end color. This will include the obligatory appeal to every modeling and prop board for the "what is the right color for the Falcon thread". To which they will reply a variety of colors and paint mixtures that will be "close" but never to your satisfaction.
- messing up several paintjobs (that everyone else thinks look perfect)
- redoing several odd bits that you just aren't happy with because your scratching kitbashings skills have imporved greatly from when you started.
- Repainting "again"
- finishing the project and thinking Damn I could do that so much better now...
- Wash, rinse, repeat... Welcome to our world of Falcon insanity...

Enjoy,
Jedi Dade
(I just couldn't resist

)