Rattiac, dad had a 54 belair conv but all we did was put on a new top. Frame was lined up.
My brothers 67 Camaro had that problem. I loosened up all the bolts and did as you described. I jacked up, (with a 2x4 chunk) at the fold point and hyper extended it, tightened everything back up. When i released the porta power jack off it settled out right.
Thats all i got for ideas.