If you have a look at the attached photo, this is the heater core mounted to the firewall when I purchased my 1951 3100 5-window. It doesn't look right. From what I've seen in photos from other trucks, the blower motor usually sticks through the firewall, filling in the big hole. Not only does this not line up with the hole, the motor isn't even deep enough to protrude from the casing.
So, in rebuilding the truck, I'm at a crossroads here. Is this possibly not the original blower motor/heater core? Could it possibly be the original unit, just not mounted correctly? I can also provide a photo of the unit out of the truck, if that helps.