Thanks hojack, that is an extremely useful piece of information. State cars and a Super 381 are about as big as it's likely to get on our layout. We run a Lionel Hiawatha and a Boucher Pacific fairly regularly, which are comparable in size and overhang. I note that your curves are slightly farther apart than a fully concentric circle due to the straight section, but thanks to your very complete information I can take that into account in calculating my track spacing.
Nobody in our Standard Gauge group is contemplating buying a Brute, as far as I know.