I looked through a Chevy parts manual.There maybe another 2 spd rear with a high gear ratio of 5.43 instead of your 6.07 for mid 50's trucks.The differential center section should be the same.Chevy 1-1/2 ton type trucks used steep gear ratios up until around 68 when the larger V-8 engines allowed the use of ratios in the low 5's.The rear wheel bolt pattern may be different or not. Rear end some later C-30 or 40 duallys may be an option.Clark made overdrive 5spds for GM trucks.I have seen them for sale in commercial truck trader swap sheets .The Clark has model numbers stamped in the case. The OD trans ID# ends in VO, and the 5th gear is usually a .80 Looks like the 264VO series is the one used ,but there are other models also.In theory, the trans should be a bolt in.I have looked at Clark trannys for my 46 1-1/2 ton, but the OD's turned out to be not OD. There was a long post last year on Clarks.
