My only concern about putting a BBC into the truck is the narrow space between the frame rails. I might suggest a 400 small block Chevy-based engine, or a 455 Buick, Olds, or Pontiac.
Yeah I know, I've heard it all about those BOP engines. But you can't beat the cubic inch displacement in a small package of one of those blocks, plus the torque is amazing. You'll approach 600 lb-ft pretty easily with a pump gas burning 455 on at least 9.5:1 compression.
I don't know why 455's were never put in pick up trucks in the 70's. They would have made an excellent truck engine.
Another really good truck engine would be a 350 tuned port engine out of a Corvette or F-body. Lots of torque for a 350, plus decent high-teens to low-20's gas mileage, depending on how it's built.
