Here's what I found in the GM Heritage site files for your other transmissions:
TC = Toledo plant - 1938 - 1-1/2ton Truck (long and short wheel base) & School Bus Chassis
PG = Saginaw plant - 1946 - 1-1/2ton Model 4502 (Chassis w/flat face cowl) & 2ton Model 6702 (Chassis w/flat face cowl) - RPO 348
Differences between the two transmission parts can be found searching through the period applicable Chevrolet Master Parts books.
Just to keep things closer to the existing 1946 truck you have, I would suggest using the PG transmission as your starting point. There are frequently 4 speed "crash box" transmissions being offered in various place on line that you may find good donor parts. There are even occasionally NOS/NORS parts that are offered. You just need to gather the parts info from the Master Parts books to get your search going.
If needed, I can offer some help on exploded diagram parts pics and part number information as I have books that cover that data. Just lets me know and will see what I can help with.
BTW - Post some pictures of the transmissions showing what parts you're currently missing so we can offer further help.
