Take a look at the Pentax 6x7. It's a sturdy beast with good optics, and at a good price on the used market -- better than the 6x4.5. Since it is built like a 35mm on steroids you will feel right at home handeling-wise. For myself, I like the 6x7 format better than the 6x4.5. Personally I am shooting 6x6, but it seems to me that you ruled out the square alternatives.

-- MW