I have had ver little luck with that situation so my advice is taken FWIW but I would attempt to heat the fin with propane then wedge some wood between the lower fins and gently bend upward . This where I usually get impatient ,crack the fin and weld it back on .But hey , it gives me an excuse to sand and polish things .
I finally got to do this tonight. I cut hardwood wedges about 1/4 inch thick and slid one in across from the front set of fins to the rear - under the bent fin. I tapped it in lightly until the wedge got a bit tight, then with a oxy/acetylen torch (small brazing tip, small flame) lightly heated the bent area. Tapping the wedge I could feel the heat had loosened it some. Kept tapping and heating a dozen times or so until the wedge was completely under the fin. I had to change to a slightly thicker wedge and continued the process until the fin was straight. No cracks! Thanks. Al.