@Smokin Joe @sneakypete Years ago I looked in building a custom .45. It is actually a fairly simple process. Purchase the "lower" sign the paperwork and then decide on what you want for the rest. At any decent gun show you can pretty much purchase everything you need. The only difficult part would be if you wanted to do porting to put on a compensator if you were building a competition rig, but doable if you have a good drill press and are willing to get your hands dirty with a die grinder of Foredom.
@verga Well,it is a little more complex than that. Not all parts are of equal quality,and in MY opinion,if you are building a custom 1911,you are tailoring it to run a specific load.
For example,my personal Combat Commander carry gun won't even cycle the slide enough to jam with military ball ammo,but with my handloads FOR THAT SPECIFIC WEAPON,it functions flawlessly with 200 grain bullets and will shoot the center out of a bullseye target.
I have a stock 1911 for shooting standard loads,and another full-sized 1911 that is hand-fitting that also shoots and functions perfectly with standard loads.
"Function ABOVE ALL" are words to live by. Literally.