Done a few. Best procedure is to grind the areas where the fenders attach to the body. Apply some panel adehisve and bolt the fenders on. Once the panel adhesive has set up, put a tack weld about every 3" on the seam.... Then, after sufficient cooling, a stitch weld in between these tacks. Then, allowing the previous welds to cool, keep stitching until it's welded solid. Don't try to run beads, it just distorts everything.. Grind the welds down, install a thin layer of filler with fiberglass in it (much stronger), let it dry, then sand it and proceed with filler as you would anywhere else on the car.... Pics in my gallery of a '36 Plymouth I'm building with the fenders smoothed....

PS--Somebody will probably come along and tell you not to use filler..... If they have the time to come metal finish the seams, invite them over to do it!!!!