Without photos it is hard to even guess. A previous owner might have changed a frame cross-member from a later truck and placed it in an odd location (and also moved the from cross member rearward)?

Who knows - the engine might have been mounted further back so that the later engine's water pump did not interfere with the radiator.

What is the engine serial number (it is pressed into the block just to rear of where the distributor goes into the block)?