I think I agree with Lee, that the best way to go about getting a Potomac would be to start with the Bachmann 4-8-4, both because if the drive and because unless you're going to scratchbuild your own boiler, the Bachmann shell would priobabaly be your best starting point with respect to general proportions. You would have to scrape all the detail, add a new smokebox door, stack, cab and domes, but it would at least save you a step.
Does anyone know what the Kato drivers scale out to? I am wondering if they might be suitable for a number of 4-8-4's with disc drivers in the 75" range, like Missouri Pacific N-75's, CB&Q O-5's, Soo's Lima quartet, Frisco 4500's, or some of RF&P's Governors and Statesmen.
The thought I have on those kitbashes would be to use a ConCor S-2 or LL/Walthers 2-8-4 shell as the base, since these locomotives had roughly similar boiler.
