74 Charger Heater

I am changing my 74 charger over from air to non air can someone tell me what is innercahngeable I got one unit that was marked 71 thru 74 but it does not lineup at all
Thanks for any assist
Bill

I don't think anything is really interchangeable. Holes in the firewall are different on the a/c cars, heater controls totally different, guts inside the heater box are all different.