Inliners International Forums General Discussion Drive Train Adapting a 1955 open drivetrain

I think you are on the right track. That'll make things easier. The T-5 conversion will be easier. If a 3 speed OD would work for you you have some options there too.

MUNCIE (Light Duty) 1940-63 Cars, 1940-68 Light Duty Trucks: Parts List 74
Cast iron case and extension housing with 4-bolt side cover. Has non-synchro 1st and a "barrel" type 2/3 assembly. Units from 1940-54 have enclosed driveline, otherwise known as a "torque tube". Those from 1955-68 may have electric overdrive. Some of 1964-65 units have larger gears with the same tooth count as the 1940-68 units.

SAGINAW 1966-81 Cars, 1966-87 Light Duty Trucks: Parts List 92
Cast iron case and extension housing with 7-bolt side cover. Fully synchronized with 30 tooth synchro rings. 1966-69 units may have electric overdrive. It has a four step cluster and all the gears are helical cut.

There are other choices too depending on what works for you. The Saginaw 3 speed OD can take the gears & side plate from a Saginaw 4 speed. There is also a later 3speed with a 4th OD. The '55 trans you have is the Munci and there is an OD version of that. Not the best because it is not sincere into 1st but easy. The Od would be Borg Warner R10. They are old school but fun to drive and can be wired to give you 5 or even 6 forward gears. They take some study. The T-5 may be easier.