So, I made a table base for a client that looks like an upside down rectangular hipped roof (out of 3/16" plate...heavy). The pieces were laser cut and fit up great. I stitch welded the inside and then welded and ground down all the outside seams to get that "seamless look". However, the two long sides that are to be welded to a "rim" around the table have bowed. Not sure how as I only welded a few inches at a time on the seams...and moved all over the place. However, it is still bowed.

I clamped a piece of 3/4" Square Bar to the thing and it almost straightened it out. The rim was supposed to be 3/4" x 1/8" thick, but even after clamping it up straight, I think it will bend. My question is this. Would 3/8" x 1" flat bar resist bending in the 1" direction (over 7.5') versus the 3/4" square bar? I know it has less material, but I tend to think that thinner material bends less in the long direction if you can keep it from twisting. I don't have a way to heat the sheet metal to take the bow out, so my only option is to clamp it up and make the rim a little taller and thicker than originally planned. However, I want to try with a valid attempt so looking for suggestions on which steel to buy to make the rim.

I DO have a hand (plumber's) torch with MAPP gas, but not sure that will heat enough to bend it back straight.

Thanks for the help in advance.