As far as I know the GM SM420 was used in all Chevy and GMC trucks from 1/2 ton up to 2 1/2 ton. There was an option for Chevy and GMC 1 1/2 to 2 1/2 ton for a Clark 5 speed. Three ton and up had 5 speed either the V or VO models (V) being direct drive and the (VO) being Overdrive models. i.e 205V or 205VO and 207V or 207VO.
My 1950, 2 ton has a SM420 with e-brake cables to rear axle as does my 1948 1 ton, while my 3 ton has Clark 5 speed with drive-line drum e-brake.
Before 1947 1/2 GM used the non-synchronized 4 speed SM420, as a new feature in mid 1947 (about May)the SM420 added the synchronized 4 speed, but still called an SM420.
If you want to stay specific to you year model go with per 1947, if you want a smother shift then go with the post 1947 1/2 (1948 - 1962) model. they bolt the same and take the same clutch. Only difference is internal to the gear box.
Also, there are a lot of parts and kits around for the later model synchro style.
Oops...back then Chevy did not build anything larger then a 2 ton. Or so I have been lead to believe.
Good Luck...happy motoring.
Last edited by GMCPic; 08/08/2014 8:30 AM.
