Bellhousing?

I'm looking for a new bellhousing, the one in my car was cut and welded for some reason. I have been doing some research looking on Brewer's website and there are 3 or 4 different 10.5" bellhousings for A-bodies all based on year, what is the difference between the bellhousings, is it just the bearing retainer hole or is there some other difference?

Is there a reason I can't easily put a 1969-71 bellhousing into a 1976 Duster?