I have some cast metal underframes that seem to match the Athearn heavy duty flat. I have the impression they were done by MDC.
The Red Ball kit is of a riveted car, not welded. That, of course, is a good thing--more variety. I think it (the cast sides, anyway) matches well to an NYC car, but it's been awhile.
For the Athearn car, another problem is the truck centers. On the prototype, the axle spacing is equal for all four axles of each truck--not so on the Athearn. When I worked on mine, I gave up and built a new span bolster--something I recommend to cover this problem and the ones Tony pointed out.