Mainly the heads. There are other subtle differences, though. On the truck engine, the heads don't have an exhaust crossover, they use coolant instead. They are also cast much heavier with more metal. I had mine upgraded with 392 size valves. Also on SOME truck and industrial engines there is what's called an extended service block which is cast extra thick so they can be bored and bored and bored. Mine is such a block, yet it is only a fresh .030 over. It is a Chrysler Firepower engine, even though it was originally in a 56 D500 dump truck. The "555 triple nickel" heads and truck and industrial heads are best for flow because their intake ports are shorter and much straighter than the 392 heads. The early 331 heads from 51-53 are the worst.
