Is it a Manual Trans or does it use the venerable 4 speed Hydramatic trans?
There is a lot of good info on the web for the Hydramatic & how to build it like one of the old "Hydro stick" Transmissions.
If the truck has the 9.3" Pontiac/Olds rear in it
FABCRAFT METAL WORKS
They make parts & has parts available for the 57 thru 64 9.3" rear end.
The shop specializes in old Pontiac parts.