Different drive shaft?

not sure but I think
7 1/4 to 8 1/4 is minus 1 1/2 inches
7 1/4 to 8 3/4 is minus 2 inches
also 7 1/4 uses 3 1/2 inch shock plates and U-bolts
8 1/4 and 8 3/4 use 4 inch shock plates and U-bolts