A500 all the way....

1) A500/42RH has the low first/2nd vs. an older 904 (2.74/1.54 vs. 2.45/1.45)

2) A500/42RH has a lockup torque converter to help drop cruise RPM

3) A500/42RH has a .69 OD ratio vs. .78

4) newer 42RH's have an improved converter hub design that is much less likely to crack (and that I'd recommend looking for)

IIRC on later B's, with some trimming of the ribs on the OD tail section, there is no floor pan/tbar crossmember cutting required, but you'll have to make your own lower bolt on mount/crossmember