Chad.S
Oldtimer
I would hold a heavy dolly on the back side of the weld and hammer it from the outside in to stretch out the weld seam. You may want to lightly hit it with a grinder first to make it more uniform and to get rid of any high spots.. Leave some of the weld though because you want to leave some metal to push back into the panel to help stretch it out.. I've actually used a ball pein hammer for this in the past when you need a lot of stretch, but grind the outter edges of the hammer head to make sure it's smooth. I've since switched to tig welding and it has helped me out quite a bit.