Sorry Jim I may have slightly overreacted
It has been a couple of years since I took a good look at a 73-76 A/C firewall and I just remembered it wrong I guess, nobodies perfect.
I think my 72 Dart has the same firewall as the 67-71 but I will have to check tomorrow. It doesn't seem like it would be that hard though to interchange heaters between years with a little time, observing and of course work.
Greg, I would really like to see the picts of the filler piece you made because I am leaning more towards this option just in case I ever want to reinstall the A/C.
Joe