Steve, your models always look amazing
Other than the number of side panels and side sill, a big difference in these 2 cars (SAL vs PRR) is the roof/side joint. The SAL cars meet evenly where the PRR X31a cars are formed like the brim on a hat (as on the Lionel model):
Now the PRR X31c double-door cars may be closer to the SAL DD cars, but I'd have to do a thorough check: