Appreciate the replies gents. Here's what worked.
I cut a piece of 3/4 plywood in a semi circle (approx. 3' long and 8" in width) to fit inside the top of the frame that wasn't bent. This provided me with a flat surface to rest the bottle jack on as well as stability to the top of the frame. My thinking was, if I make the semi circle span 3' it will provide lots of strength and won't bend when I start jacking the bend out on the adjacent end of the frame. Sure enough the bent area started to go back into position and the other end when the semi circle was stayed true to shape. No heat required. I then positioned it in my press to remove a small bend in the outer flange and honestly you can't even tell the sign was ever bent. I should have taken some photos as it worked very well.
