Jeff, the transmission is a Muncie not a Saginaw. The SM stands for synchro mesh.
The transmission you have is a '48 or later, and so is the bellhousing. The 'correct' transmission for your truck did not have the synchros and its not uncommon to find that someone changed both transmission and bellhousing to get the synchromesh transmission.
The original bellhousing did have some bosses to mount linkages, and the mess you're seeing now is someone's effort to adapt all that.
Some of what you describe does not make sense to me either, as Cletus said. Some of the changes seem a bit backward from what I'd expect.
On a little later truck, the Advance Design/New Design trucks, the only difference between a Chevy and a GMC frame is the front crossmember. I don't know the earlier trucks as well but would expect the crossmember to be a key difference.
If you are serious about that word 'restoration' 228's aren't really very hard to find, and 248's, 270's and 302's would all bolt up in exactly the same spot and look much closer to original than a 235.
Personally I'm not picky about restoration and would tend toward a larger GMC motor if I had your truck.
