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.
I'm pretty sure that Chevy changed the bolt pattern to the bell housing in 1948. Before that, the pattern was smaller, so 1948 and later transmissions won't fit a 1947 and earlier Chevy.
Maybe things are different for GMC.
