Bill, although mine does not have A/C, it took me all of an hour to replace my heater core, including complete disassembly of the heater box, MAKING a new right hand air box door seal cleaning everything up like new, a coat of paint and reinstalling it. I've done the A/C style boxes the same way. "It ain't hard" to fix it right. Now, I will say this. When I do one, I take the front seat out. "THAT" makes it a lot easier.
