I would suggest cutting it out and starting again from that point. Heating it up could just warp it in other places as well. You might try to put weight on top of the frame to help hold it down while you do it next time, but make sure its square and well supported from below before you lay weight on it.

When I built my frame, I Built a strong jig out of 4" square stock with various support beams holding it together in a true squared possition. I then tack welded my frame rails to the jig and it lived there until the entire botton frame section was complete before I cut it off.