You didn’t say what type of welder you’re using? MIG with gas or shield core wire, what size wire. Most welders have a chart to set up the welder for different size metal that you’re welding, this just gets you in the ball park and may need minor adjustments in wire speed or power. If I remember right that area is 18 gauge metal.
One thing that I’ve found a great help is backing up the seam with a copper backing plate. You can find some at Eastwood (hand held or magnetic) or go to the hardware store and pick up some copper tubing flatten it in a vice and have someone hold it in place when welding (make sure to wear gloves when holding it, it gets hot) You might find some additional tips on my build thread (post # 76 and more thru out the build)
http://67-72chevytrucks.com/vboard/s...=318544&page=4