Massively overblown engines run better as diesels, and vague recollections are that GM foisted off a "slightly modified" 454 as a diesel in a couple of passenger cars around that time. Are you sure the 454 in the truck wasn't a diesel? (With enough positive manifold pressure you could get diesel ignition without even changing the mechanical compression ratio, and the fuel can't burn any faster than it's injected - so who cares if it explodes instead of burning?