Well after thinking about it all night I thought I would try again to salvage the last door flange. After some attempt I bagged that idea and decided to start over. The third try actually went much better but when I tried to straighten out a small bow in the channel I screwed up the part beyond salvage. I probably should have accepted the slight bow - it really wasn't even noticeable unless you sighted down the line.
The fourth time was the charm! I decided that I would not attempt any heat corrections at all. If the channel was bowed beyond acceptance then the only option was to scrap the part. But this time it came out very nice and straight.
I figured out the method that works using a blow hammer. I made the first bend in the 36" brake by moving the foot all the way forward and clamping the part in the brake then using a blow hammer to make the first bend over the edge of the brake. For the second bend I clamped the part in between the edge of a piece of heavy steel angle and some square tubing then used the blow hammer to make the final bend.
I welded the two halves together then made the notches to fit the part to the frame. I tacked both sides to the frame. Persistance paid off!
