The spline does change (you mean the steering box side right)
The 73 up has a large sector shaft (per Firm Feel) so that part of the arm is definately different....but its also a much less expensive pitman arm....saw this info on the Firm Feel site a day or two ago .....
But..
I am searching the Firm Feel site again and for the life of me cannot find the reference I am talking about.....
I had copied it to the email I sent to Firm Feel asking about it...it said:
"NOTE : If you own a '62-'72 "B" body or '70-'72 "E" body you can save some money on your pitman arm by using a '73 & later power steering box. In 1973 all A&B&E body cars went to the "large sector" box,( The sector shaft attaches to the pitman arm). The late pitman arm is still readily available at about half the cost! You'll save roughly $75.00 over the cost of the small sector pitman, and there is no problem with installation, clearance or geometry using this large sector set-up on the cars mentioned above."
I swear to god that the reference has disappeared!