You might want to look out west for a frame, CA. and AZ might have a rust free one that could be shipped to you for less than the repair of this one. I have seen them box weld a frame for a vette that was severly rusted out. When they were done it was stronger than original. Nice car I'd try and keep it going. Think of it this way better to find out now then to find out at 80 mph!
Regards
Mark
