The floors, firewall, and inner fenders are 18 gauge. Call your local welding supply houses and ask who in your area would have some scrap metal to sell you. I bought two strips of 18 gauge, 12"x36" each, for $10 from a local metal works house. They were more then happy to sell me the "scraps".

I bought a piece of copper pipe from the local hardware store, beat it flat, and use that to go behind the butt weld seam. Works well.