The seam sealer out of a tube is not made for smoothing body panels and hiding seams, just sealing seams up. Body filler (bondo/rage/etc) would most certainly crack pretty quickly so it's probably out. I'm by no means an expert but it seems to me that you'd have to either weld them together or smooth them out with lead/body solder, or some combination of methods. Any hod rodders on here? They are probably the experts.