R Wilson asked:
"Would it be possible to just take an AAR '37 10'er, replace the roof and ends to get one of these? Is it such that the sides would have to be reduced in height?
Looking to get a prototypical model of them and want to do it right."
Tim O'Connor replied:
"Aren't they basically PRR X31's with different ends and rivet patterns?
My instinct would be to start with the Bowser and figure out how to replace the ends, and use Archer rivets for the rest."
Not really. The SAL cars are more akin to the 1932 ARA boxcar design, with different side rivet patterns (double rows, plus an odd 5-panel to the left of the door/4-panel right of the door pattern for the single door PRR cars) and tabbed side sills. You'd have to redo the sides and underframe if you start from the Bowser model, which actually would be almost as much work as starting with the old MDC model, which has an incorrect roof profile and a caricature of the Pullman carbuilders' ends. Still, it's a doable kitbash if you can come up with the ends; just not as straightforward of one as you'd think.
Sadly, the best approach would be to pony up for a Sunshine kit for the automobile cars (assuming that the original poster is working in HO scale). No real help for the single door cars except for the suggested kitbashes.