All - After getting caught up with the latest posts and information provided, I wanted to offer some updates on the gear set identified by 2ManyTrucks. Not trying to be a wise guy or anything, just identifying a minor error that needs correcting so Doug has the correct info to search for what he needs.
The GM# 3709359 7-20 tooth gear set is listed for the Passenger Car and Truck 3 Speed transmissions on page 398 of the 1929-57 Chevy Master Parts and Accessories Catalog (1st snapshot pic below) and will not work for Doug in his SM420 4 speed unfortunately.
GM# 3707598, also a 7-20 tooth speedometer gear set, is listed on page 400 of the same Catalog (2nd snapshot below) and will work with Doug's SM420. The good part of this info is that same gear set was used in the 1954-55.1 (1st Series) 1/2 ton's with 4 speeds AND (drum roll please) 1955.2-59 1/2 ton's equipped with 4 speeds and 3.90 rear gears plus also selected 1960-62 C-10's with 3.90 ratios (depending on tire options). Pictures 3 and 4 are from a 1933-62 Chevy Master Parts and Accessories Catalog that show those applications.
So the gear set Doug needs can be found in a wider group of source SM420 transmissions if all that can be found is used.
Haven't had any luck yet finding anything offered on the web for this GM# 3707598 set .
