You have 2 options...
There is an aftermarket "A/C Delete" heater box available.
I see them all the time on Evilbay. They don't look correct, but it does eliminate the A/C stuff, and covers the Big hole from the A/C heater core.
The other option, is to fab up a piece for the firewall, and cut the opening the correct size, and use the standard heater core, and box. That's what I did on my Nova, and it looks factory.