Pretty much any new roof mounted RV a/c unit will fit ok. But, all of them will need to have the roof modified to support them in your '72. The old a/c is large enough that it sit on top of the ribs both in front of the a/c and the rib directly behind the a/c. New a/c units clamp on the 14 x 14 inch roof vent opening, and your opening has absolutely no structure around it at all. Just the roof aluminum skin and the ceiling. Also, if you don't want the condensate water running down the outside fo your airsteam, then you'll need an Airstream drip pan or a drip cup option for the a/c to connect to the drain tube that runs from the current a/c down inside the wall. If you let the condensate run down the outside of the trailer, you'll end up with a nasty looking water stain within a short time.
Do you have an airstream dealer close by that you can talk to them about the drip pan? That's what we did for our new a/c. We ordered the whole kit from the local Airstream dealer.
Regarding supporting the a/c on the roof, some folks built aluminum bracing around the opening and others have used wood. We used wood, and it worked very well. Do not use treated wood as it will corrode the aluminum. The frame we made is shown in our thread at post 364 here: http://www.airforums.com/forums/f185/little-girl-refurb-50967-26.html.
Also, when you remove the old unit, you'll have a bunch of screw holes to fill. We drilled them out and used olympic rivets to fill them.
Hope this helps.