Hi Dan,
I'm a fellow tank enthusiast myself. I am fortunate enough to play with 1:1 scale tanks at the AAF Tank Museum in Danville, VA. I don't suppose you know Dave in Australia with the 1:2/5th scale M60 and Sherman? I've emailed him a few times and he seems like a really cool guy.
If you find a bone yard with several T-20s, I hope you'll be kind enough to tell us where it is after you're done picking through it. Those T-20s are not so easy to just find, not to mention all the other parts in the machines that have them. It would make a good transmission for such a project. Making the tracks counter-rotate would be very easy. What tank do you intend to build?