Kind of depends on how much you want to reduce your smaller radius. Wheel the centre much more and it'll go hard and start fighting you. I'm assuming what you want is near enough flat sides in which case I would crop an inch off either side of what you have, make a new pair of sides with the upper edges shrunk and turned over to match the cross-section you have already then weld them in.
I would spot the sides on every inch or so then knock the tops off the welds with a die-grinder. Wash it in the wheel to get your final shape then fully weld.
Don't forget, there's no right or wrong way and someone else's technique might suit your style much better.
Best of luck.
Will
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William Pointer
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