Posting this here, but it's probably for posterity. Trek 640 frame that I bought off CL and the seller shipped to me from across the country. It's a touch too small, despite my best efforts at reading the geometry from the old catalogs. It's mocked up here as a possible 650b conversion, but, like I said, I probably need to go for the next size larger. The stem is maxed out and I'm not sure even a Technomic would make it work.
