If it's not practical to get to the back with any sort of a clamping device, then I resort to holding it with a bow. I have a bunch of different length bows that I made from a high carbon wire made from 1/8" and 1/4" wire. I got some of it out of over large head cable. It’s not one of the conductors but the tension wire that is used for supporting the cable in long spans. Of course any flat spring steel or high carbon wire will work. You don't want to use to small of a heat sink or it will push the parent metal out around the weld when it gets hot. Use a three or four sq.in. stamp to spread out the pressure applied by the bow to the back side.
As an example, In the top of my 1950, 3604 I have nine 1/4" holes in the top of the fender. They are arranged in three, three hole patterns where the flashing lights and siren were mounted when the truck belonged to the fire department. This is important, I hammer a flat stamp about three sq.in. so the curvature fits the back side of the fender nice and tight, then use one of my longer bows braced against the axle or frame to hold the heat sink in place while I tig weld the holes up. Then grind and hammer it flat, you might have to stretch it a bit or shrink it some. Generally it will shrink up the area around the welds and with some light hammer and dolly work, and a little picking you can stretch it back up to grade. I have yet to try a shrinking disk but might try one when I do the major body work this winter.
Denny Graham
Sandwich, IL
