Well I went ahead and replaced the heater box due to very poor heat and almost no air flow. This past summer I grabbed this HVAC box from a 76 dart sedan at LKQ. Also grabbed a few other neat things from it like a power steering column for when I do that conversion.
I spent the first day getting the heater box in by making a template, and then cutting the necessary holes in the firewall. This is definitely not the professionals method, but it actually worked quite well! Got it positioned about as good as possible. there is still a little bit of a gap on the top round seal, but I also need to fabricate a support bracket to hold that up tighter - at least that's how it was in the Dart I took it from. Day 2 was getting the electrical and vacuum all routed and hooked up.
Not only is this heater core good, but she'll blow the pants off your female passenger now! Surprisingly, all the vacuum diaphragms work! and now it's already for A/C come next spring!