Peter,
Your question is probably over my head, but I'll jump in to say that I think this is a non-issue. Assuming your 072 is the outer curve and the 063 is your inner curve, and that you Are going to run the 21" cars on your 072 curves - any overhang would be on the outside of your 072 curve, so it should have no effect on your inner 063 track. If you're asking about the side of the 21" car, between the wheels, bulging out toward the inner track, this occurs with my 21" ESE cars, but not very much and nowhere near enough to interfere with my inner curve activity, even when I've got my VLBB on the inner track. Only caveat is that my tightest sea level mains are 081 and 072. I will say that I use Atlas track and leave over 5 1/2" between mains at every curve apex - but again, the "bulge" isn't anywhere near interfering with the overhang of the VLBB.
In terms of how 21" and 18" cars look together - when they are on the same train, I am not a fan of the look (but I'm an artist at heart and aesthetics are very obvious to me). But if they are operating next to each other, it's not an issue. Even the passenger trains used in my local (real-life) excursion trains aren't the same length.
Peter
