Heater dilemma

I bought a '66 Barracuda a few months ago. It was all apart. I knew I'd be eventually needing to be chasing missing parts, but...
The car turns out to be a factory A/C car & the heater that came with it is a Non-A/C heater. The hole for the heater motor in the firewall is considerably larger than the hole used by the heater that came with the car. The holes for the hoses to the heater core from the engine match up OK, but the motor doesn't. I don't have the $$ to get a working factory A/C unit, so I'd like to use the heater I have, but I'd need to make some kind of adapter to mate it up to the firewall. Has anyone run into this before? If there's an easier/cheaper solution, I'm all ears. I can't do a 'heater delete'. I need some heat & defrosting now & then. Thanks!

Heater hole.jpg

Heater firewall inside.jpg

Heater front 741.jpg