Installing inner fender, 64 Barracuda

I don't see why you couldn't dolly it as flat as possible on both pieces prior to welding. Your problem may be that as you weld the panel may distort some and throw off the alignment. It makes more holes to weld up but I usually use a LOT of sheet metal screws to hold everything in place and where possible clamp on both sides of the area I'm welding on. After welding, grind down the welds and repair any that look weak then seam seal the joint and use a skim coat of filler over the welds to clean up the area.

Larry