wheels and rear end questions.

[1967j]

hey I was wondering if any of you could help me with some questions, first I was wondering why newer rims dont fit on the old trucks, i want to put 20's on but when i emailed american racing if they fit they said they wouldent, now i belive it has something to do with the wheel bearing and such? is there any way to fit that to fit newer rims on? I plan on putting on drop spindles and pulling the disc brakes out of a 72 truck. also what trucks had a stock posi rear end i could put into my truck or what do i look for when at the wreckers for rear ends pref. 373, my 411's are abit tough on the tires and gas :P