I know that the 4 speeds in the 39's were built by both Saginaw and Muncie. I believe the four speed was referred to as a SM-22? It is the non-snycro and was used for a long time. It is the same size as the 37-46 3 speed and is a fairly simple drop in for those year trucks. I would think the same might be true for some of the pre 37 trucks.
Don't know if that helps or not, but just a few thoughts. For the late thirties, standard equipment was the three speed for 1/2 and 3/4 tons. Big bolts got the four speed. However, it was an option to get the 4 speed in either the 1/2 or 3/4 ton's and lot appear to have been equiped that way.
Gearing on the 4 is meant for hauling. First is 7.23 to 1. 2-4 nearly approximate the three speed with 1:1 at the high gear. Not exactly the kind of gear to spin the tires with. Since I'm doing mostly improved road driving, I rarely use it. Since you've got the big bolt, you probably would use it more that I do!
I would have added these thoughts earlier, but it really just came into my mind now.
Let us know how the swap out goes.
