Those Pontiac OHC sixes are beautiful looking, unfortunately everyone goes for V-8s because they fit so well in the old Pontiac cars. The big Pontiac engines came in 400, 428, and 455 in the 70's. I think all the GM divisions had a 455, and Cadillac had the 500.
They Pontiac 6s were 250ci in the last couple of years before the muscle-car wars caused Pontiac to drop them. It's a shame, really; they had a 6500 RPM redline. There's a forum out there devoted to them. If I found one I'd pick it up then find something to use it in just for the uniqueness.
I like the old Kaiser Tornados too, but they're a different breed. They were in the FSJs early on, and they are really good engines. They get a bad rap because of their uniqueness and some of the applications they were used in.
The Tornado had a lot of torque and a low fuel consumption, all while driving both valves off the same cam lobe. It had 6 lobes instead of 12. They were put in a lot of military vehicles for years after they were discontinued in the consumer market, and were maybe a little underpowered for some of those.
But I'm drifting a bit. I'd love to see the finished product and construction photos. I love to see unique swaps, and I think that the OHC would make a good choice. As far as transmission selection, a 700r4 or an NV/NP would probably work, they were used in GMs and Jeeps so you may get lucky and find all the parts at the junkyard.