Thanks, i will cut the whole thing with a cutting disc then use grinding disc and then the dremel for the finer bits. I think i just need to go for it. Im just not sure about those little lugs you can see in the far right of the corner. The new valance panel has them im not too sure if i should cut the old one fully off or i have to try and but up to them?
The new valance should fit between the flanges, then plug weld them together (or use a spot welder if you have one) and cover with a U piece spot welded top and bottom. The top of the valance needs to be welded too where the seal fits and if you plug weld bear in mind that the weld will need to be cleaned up so the seal fits properly. I use the edge of a new disc in the angle grinder but as you seem keen to use a Dremel you could spend an hour or two using that.
the pic ive shown is front whheel arch they tend to have more spot welds than any other panel , the rear panels will have spot welds roughly every 2 3 inches so will be easier