also, if you get the late W56 you'd have to get either a matching case or swap the shift rods from a late model case. both the G and W series were available in forward shift versions, but the G series rods are too long to fit in the late W56
and you say you have a 4 speed currently? remember, the 4 speeds and early 5 speeds(L43, L45, L50, L52) are shorter than the late 5 speeds like the W and G series by about 4", so you'll have to have your drivelines modified and possibly relocate your crossmember
