How bad is the frame rusted? If it is really bad, there are weld on frame pieces that can be bought to replace rotted parts. I would check with Year One.
To increase the strength of the unibody design you should also look into some subframe connectors from places like Competition Engineering or South Side Machine. These aren't part of the original frame, but tie the front and rear subframes together to make a more rigid piece.
[This message has been edited by boodlefoof (edited 03-20-2001).]