Todd - Did we ever decode the casting numbers/date codes on your existing transmission? I know we did for the later 1955 SM318 that doesn't support your torque tube drive.
Reason I ask is there was a design change of the internals of the SM318, from what I can tell searching through the Chevrolet Data files. In 1953 the SM318 3 speeds switched from the 1948-52 single row of roller bearings design between the clutch gear and main shaft, to the double row (14 thick and 24 thin) roller bearing design setup. There were other internal part changes, like the gears which prevent the '53 and later pieces form swapping with the '52 and earlier parts.
Below are screen shots from the GM Heritage center files showing the needle bearing transition.
I would also mention the 1954 Chevy Truck Shop Manual disassembly/reassembly instructions for the 3 speed describe the dual row of needle bearings in the service/repair instructions guidance. They can be found
in step "6" of this page from the manual.
Perhaps you can post the two transmission casting numbers details so we can try and decode them and confirm what they actually are?
