will new clutch fit bellhousing?

The only difference between a 10.5" 513 bell and the orig 10.5" A bb 372 bell is the retainer bore size and 513 is aluminum instead of cast iron. As long as it has the right fork pivot inside, all the BB a-body mechanical linkage will bolt right up correctly.
well t hat might be reassuring - so why does retainer bore size vary? this stuff new to me obviously, but can retainer be replaced if needed?