Question about '48-49 dodge p/u

My neighbor has one setting in the treeline and is in better shape than what I thought it would be. It has a flat head 6 and of course when parked the air cleaner was removed so the motor could fill with dirt and rat droppings. The motor is mated to a standard trans. and just curious what other engine would drop in there? I know, not enough info, but I've only seen it breifly and plan on looking again.