I'm stumped!

OK, I'm slowly making progress putting together a 1966 Barracuda that I bought in disassembled condition. There were a whole lot of '66 Barracuda parts that came with it & I was hoping I could figure out what went where. The car was originally factory air conditioned - the build sheet says so & it even has the AIRTEMP decal on the back window. There is no other sign of A/C parts anywhere....but there IS a complete heater box/assembly. My plan was to just go with the heater....at least for now. My problem is that it doesn't line up with anything on the firewall where it would attach. I stuck the two tubes through the firewall & figured SOMETHING would line up, but nothing does. Also the firewall hole is 6" in diameter & seems WAY too big for the heater. Does an A/C car have a bigger hole in the firewall than a non- A/C car?
Here's a couple of pictures. The heater I have has a '741' stamped in red. I've got a feeling I'll be needing another heater, but I thought I'd ask here first.

Heater hole.jpg

Heater back (firewall).jpg

Heater front 741.jpg

Heater firewall inside.jpg