swaping ac unit. Help!

I should be fairly straight forward and I don't see why not as long as you have all of the parts from the 72 to use. I'd make sure that you get pictures of the firewall holes and measure everything out before you start.
I'd say it's a fair amount of work:
1. Under dash A/C box = pain to remove and install.
2. Condenser in the front = whole grill and hood latch panel has to be removed.
3. Holes need to be cut into the firewall = scary :D
4. Bracketry has to be swapped from 72 motor to 67 motor = ok if its from v8-v8, problematic if its from /6-v8 or vise versa.
5. A/C controls need to be swapped out = need to be careful with the vacuum actuators and such as they are probably very brittle

I've never done it, but my car has A/C and I'm just thinking through all of the "moving" parts :)

Charlie_S is a good resource for A/C questions.

Welcome aboard BTW!