back in the days, the 100 amps alt was called Leece neville alts which uses a quite diff alternator bracket setup. Is not really about the output but the diff manufacturer alternator( pic posted )

At this moment you can get 100 and up to 130 amps alt from tuff stuff, and several other manufacturers on the same casing of the regular alts so keeping the existant brackets. These newer alts should be on around 60 amps rate at iddle which should be quite enough. Sooooo I wouldn't take care about the brackets for 100 amps Leece neville alts unless you reall want those for whatever reason.

you can get even a 78 amps stock alts from earlier to mids 80s which will fit nice too and works, on stock replacemenet price rate. they are A BIT wider, maybe couple of mm, so fits a bit tighter. Is the one I'm running ( driver with AC ) and I trimmed a bit the rear spacer and got a bit longer belts to sway the rear of alt from block.