Steve:
I have a 58 cm Atlantis that I bought new in 2003 (sorry, no, it's not for sale), so I've put a lot of time & mileage on it. On a visit to RBWHQ, I spent a couple of hours on a 53 cm MIT Atlantis. With only seat height adjustment it fit fine. It rode noticeably more comfortably than mine (which rides pretty well) and was especially impressive on some trails we took. It seemed to take switchbacks as easily as my bike, despite the longer chainstays. The tires were nothing extraordinary, just whatever Rivendell uses normally.
Were I in your situation, I would pull the trigger on a new one & not grow wistful over the mystique of an earlier design. There's been discussions about Toyo vs Waterford vs Taiwan, but IMHO it's all pretty academic. There's a limit to how well a frame can be brazed, and Grant / Rivendell has a stellar record on design & tubing selection. You'll have advantage of the latest thinking in a brand new bike. Go for it.
dougP