I would agree with 78Buckshot on the small block. Your rear end ratio (assuming the 2-speed) is pretty low. My 1958 6400 (same rear, I think) 1with 9R22.5 tires spins the engine at 4200rpm at 70mph on the freeway. 3600 @60mph. I am running the tires 327 that came in it. I use it for towing my 35' travel trailer and everything else. I don't think you are going to want to spin a diesel that fast so, if you go with a diesel, you may need to change the rear end out as well.
