This is just my guess; 72 was the end of a body style run. 73 had a completely different heating system. I doubt that Ford would have changed anything for 1 year.
Thanks, I didn't want to get the wrong cobanation for my set up. I'm sure glad that their are good people on Fordification, that take the time to read, and answer when a fellow bumper needs help..
Those who do not learn the lessons of history, are doomed to repeat it.