I don't think the units line up. The fan on the A/C equipped cars is completely under the dash while on the non A/C cars it is protruding through the firewall into the engine compartment. Therefore, I believe the units are entirely different. I have a 67 that is in the same situation. The A/C is torn out and the existing heater unit lacks performance. There's a company called Flex-a-lite that offers a product called the Mojave Heater. It get's heat from the cooling system and pushes out 140 CFM of air. The best part is that it's only 10x9x5 in dimension. Should clean the under dash area nicely. You can find it offered in Jegs or Summit racing catalogs.
I have converted several cars the other direction, going from the factory heater only system to factory air. Either will bolt into the same place, there will be a small oval hole in your firewall to plug. A piece of 2 inch air conditioner aluminum tape would cover it easily.