I would not use anything less than 16 gauge(2mm), but best bet would most likely be a scrappy car where they are intact, if you can find one that is.
I have seen a "pro" shop weld a seat using round tube as the only support between seat and floor on a track car, no reinforcing or anything, then the first time a fat c**t sat in the seat it broke through the floor, but I promised I would say it was ROOSIE, shit too late!!
tabetha
nb roosie did not have anything to do with the welding