Yes, it is possible but boy, it is a PITA. I've done it twice and swore it is simply easier to swap out the earlier 65 (two speed heater) for the later (three speed heater) underdash harness.
You'll also may have to change out the engine gauge feed harness (I believe) due to the wiring changes on the engine side for the heater, but maybe not. I"d have to do some research on that to be sure.