Different Heater Boxes

The firewall will be different with any model non a/c box. 73 and later uses a totally different controller but so does the non a/c controller for pre 73 ( you have push buttons now ). Some moding req'd no matter which way you turn.
I would prefer the earlier non a/c box and controller just because its all mechanical, no vacuum required. Controller and bezel for that should be easy enough to find. Good luck