A quick update:
I got the overdrive unit removed from the main transmission housing. There is no internal trickery...it was just that the gasket really had the housings stuck together. I can see that a challenge is going to be keeping the roller bearings in place when removing and installing the overdrive unit in my truck. They are not captive in a race or anything. Consequently, I'm making some locating pins from 6" bolts, so that I can slide the overdrives straight off and back on. Hopefully the gear oil will provide enough stiction to retain the bearings in place. I'll try and post photos of everything tomorrow.
I took note of the casting numbers on the housings of the transmissions and overdrive units. I can find no comprehensive online listing of transmission casting numbers. By perusing different tri-five, chevy, and camaro forums, I was able to discern that both transmissions are Saginaws truck transmissions, with build dates sometime between 1958-1965. I cannot yet resolve it better than that. It is quite possible that the trans currently in the truck is original.
-Kevin
