That would make it a Clark. They were popular for use in soft ground because you could rock the vehicle back and forth to get out of a mud hole. It doesn't make much difference what the truck had in it, but it would certainly be nice to start out with the correct clutch housing and front engine mount to install a Pontiac. Does it have an ID tag on the door jamb. That will tell you if it was born with a 6 or V8.