That should work, but I would cut away most if not all of the flange and weld the new metal at the horizontal to vertical bend. Welding from the underside should minimize blowouts. That way you'd have a rust free flange to bolt down.
I didn't realize that the flange laid on top of the wood and not underneath it. Probably makes sense that it was on top though, as it would probably have been way rustier if under the wood, and trapping crud and moisture there.
I’m thinking that if I cut away the old flange, then I’d be forced to weld the entire outer perimeter (3 sides) of the wheel house. I’d like to avoid that much welding. If I just tack welded the new flange (old flange cut away) then I’d have voids between tack welds. I suppose I could buy seam sealer to fill the voids.
