A couple things to watch for..
the full radius will lead to leaving tracking marks in the panel. Over at Metalmeet.com, they have the formulas for how wide the flats should be, should you decide to re-machine them. It really improves the work surface. Its not IMPERITIVE, but if you are trying to achieve "No Bondo", then you will have a rough go of it.
Springyness: On my wheel (Imperial 26F), I can re-insert about any time. I get MAYBE .020 total deflection. When you move up to "fractional deflection" (Measurable on a tape measure, my own terminology) , it will cause what you are running in to. If I were making the piece you showed in the pic, I would first hammer it with a mallet and a shot bag to stretch it out faster. Then tuck shrnk the sides a bit. Insert it into my wheel, to knock the lumps out. I would crank my wheel down as far as I could, while still being able to move the metal. About 2-3 passes, and I would re-tighten, make a few more passes , retighten, and so forth until its smooth. That piece, as described above would really only be about 15 minutes total time, including tuck shrinking the sides a bit, so you don't overstretch the metal.
Something cool to know. The finish of the wheels, is the final finish on your work piece, so polish your wheels! (Make an arbor, and do in the drill press, not in your lap! Otherwise, you'll make unintensional flat spots)
DO NOT try to plannish/wheel mig welds
DO NOT try to wheel over lazer cut edges
DO NOT spin the wheels and try to insert the piece like you see on Biker Buildoff.
All are guaranteed
to leave marks in your dies.