Shift manual transmission into neutral. Remove rocker assembly (as was suggested before) or remove pushrods. Check that lifters spin or move and are not stuck in the galley. Remove the starter, distributor, spark plugs and loosen fan belt. If the engine still doesn't rotate it needs to be removed from the truck. Remove the timing gear cover and gear and see if the cam can be turned. If the crank doesn't turn, for a rebuilt engine, it needs to be taken apart to see why. If this was an engine from a worn out old field truck, I'd put a 6 foot lever on the end of the engine and go for it.
