The big frame 100A alternator uses its own brackets. It is physically bigger, heavier and belt adjustment travels in a bigger arc vs the standard roundback/squareback. I have both setups for a BB and alternators will not interchange.

There are plenty of high output squarebacks out there as mentioned... still idle output on these is a bit weak. The big case 100A has great idle output which is why I run it... even though it is heavy.