I had a '76 corvette built up for over 400 hp - it turned ~12.6 in the 1/4. I've seen pictures of a Dodge 1 ton dually pulling 11.6 in the same 1/4! My car was VERY fast - the truck was over double the weight and still much faster. I was told by a Cummins employee that 750 ft lbs is possible from that 4B, along with 250-300 hp. The marine 4B typically pulls 250 hp, from what I've seen. The Isuzu is supposed to be a good choice as well.
If you can get a 6B that cheaply with that few miles on it - I would not hesitate. Will it fit? And will it pull the frame apart if worked? Those are serious engines!
