GMC lists 4 different input shafts for the SM 420 I don't know what the difference is. I own several 350 GMC with a SM420 as well as one ton and 3/4 ton pickups.

https://photos.app.goo.gl/voCAihTogync4tNi7

In looking through the Hollander, it appears that 1948-1959 SM 420 is the same, but it was different when it was used behind V6 GMC and V8 Chev in 1960-1962. It also looks like it applies to 1958-1959 Chev with a 348. These would be very rare because most trucks this size wold have had a 5 speed.

I trust the GMC master parts book and Hollander than I trust Novak's information.