Have you considered gluing the top on? I welded on 2 tops on my 32 sedan and warped them both. The third time I glued the top on and it look good and is strong. I talked to a 3M rep and said as long as I tacked the corners and glued the rest, they would stand behind the job. First I dry fitted the piece and secured with sheet metal screws every 2". Then I took apart and ground off the burrs and then roughed up the mating surfaces with 30 grit grinding wheel. Then I applied the glue ,except for the corners, and reapplied the top using the screws back in the holes. Snugged them up untill the glue started to ooze out and let cure.Next day I removed all screws and ground off the excess dried glue. Then I used a bevel bit in my 90 degreein each screw hole. then dabbed a little glue in the beveled ho;es and let it set. Then grind off smooth and do the body work. Been on there for 3-4 years now without any failure. Just a thought. Later