Updating this for the sake of documentation: finished the brakes and got around to checking out the transmission.
Turns out it's the SM318 that I suspected. Additionally, it looks like the clutch size is a non-issue for now. Opened up the inspection cover and the clutch seemed to have a good amount of life left on it, and the throw out bearing turned smooth as butter. We'll see how well the transmission likes to work when I get to take the truck on a test drive.
Thanks again to Quigley and James.
^(Not sure if above photo will display properly or not, so apologies if it's just a broken mess.)