Ok alex so it sounds like you have a sm420 4 speed transmission the sm420 and its younger brother the sm465 you can still get parts for and really as long as the case is good I believe you can replace just about everything in them. As for seals in other places like the axle the rebuild manual for the eaton axle I am putting under my truck says to use gasket maker. For the vacuum 2 speed I don't know if you can get a new diaphragm for the 2 speed unit but I have seen people replace them with eletric shift units as you can still buy those so you might be able to find one if the 2 speed goes out
1967 Chevy c50 327 w/ sm465 5000Ib front 15k 2 speed rear w/ 22k springs, single axle, hydraulic brakes
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