You know, if you're V8-ing the truck anyway, you might consider a BOP 455 engine. Small as a 350 but torquey as a 454. They weigh considerably less than a Chevy big block too. With the right cam, and a good compression ratio, you should be able to get 600+ lb-ft out of them. I've even seen BOP 455's approach 700 lb-ft at under 2000 rpm. That's pretty good torque and should make an outstanding tow motor.
...Muncies will bolt right up to them too, as well as a host of other transmissions. Richmond and Tremecs to name a few.
