The rear springs are different between '66 and '67 too. "66 has a large coil on top and '67 has a small coil. The '66 frames did go to the '67 spring later in the production year. If you have a reputable frame shop in the area, take it to them and they can set it up and check it for twist, diamond, etc and straighten as needed. We do this where I work and a bare frame is easy to get straight, unless it is very bent.
What is so bad about yours ? Is it rust damaged or just bent ?