We need to know a little more about your engine. Chevy didn't build V6s in the 67/72 period, but GMC did. Are you sure it's not an L6? Like a 250 or 292 inline?
Anyway in the 70 model year, all GM's transmissions: 3 speed Saginaw, 4 speed Muncie 465 (manual) and TH 350 and TH400 (automatic), would bolt up to any Chevy L6 or V8. Only difference being that manual transmissions require a bell housing. I don't know much about the GMC V6 305, but I've heard it would climb up a phone pole if you put it in granny gear. I would assume it too would bolt up to any other Xmsn of the period, but I don't know. As for the later Chevy V6s, I hear they're just 3/4 of an SBC block, so why not? As for the Th700r4, I also hear it's the same size and fit as a TH350, so it ought to work. Maybe some experts can prove me wrong.
