I just got done making quarters for a 1946 chev panel that were 7 foot long . I cut out the inner panels to access the back of the weld so as to hammer and dolley the weld seam to STRETCH back out the shrunken weld line . I even came up with a slick dolley for the inside made from some metal and a piece of leaf spring . I then took a piece of 10 ft long 5/16 rod with a hoop made in the middle so that one half was inside the truck and the other was outside with a small 90 bent to fit in hole I drilled in the dolley . This allowed a helper to go inside the vehicle and push out on the dolley with the rod on it to act like a bullseye so that I knew exactly were to hit and stretch the weld . I also drilled a hole for my air chissel in the center of the dolley and had my helper on the outside this time with a large flat dolley to follow along as I was able to push out the sunken weld line as well as do some planishing as well . This all done after cutting down the pride of the weld on both inside and out as mentioned above .