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1/4", about twice what you have by the looks of it. There is no need to butt weld where your attaching the rocker, except the last 2 flats that goes to/under the cab corner/side panel. If you cut the new rocker (as low as possible) line it up, draw a line around it on the old panel, then cut the old panel off 3/8" or so to the inward side, this will leave a "backer" to weld to. On the sharp corners, (once you have a few tacks on) just cut through the old rocker at the edge of the new rocker 1/4" or so going each way from the corner & tap the new panel downward a little & weld it up. I do as much of the welding from the bottom as possible (bottom lip, going up the back-inner edge of the cab corner ect.) welding & grinding it isn't as much fun, but no holes to fill & finish. Have Fun! Lorne
Ball Peen??? Don't you mean body hammer??that you should be able to encourage back even with it just screwed in place. A little relief cut down the high point and inch or two might help as well. then just stitch it back closed while welding.
I wouldn't cut it off. I would slice it down enough to push it back. The plug weld the face of the patch to it. You should invest in a body hammer set, with some dollies. Well worth the money.