For a 79 Spider 2000? I've got some pretty nasty rust on the floorboards (or where parts of the floorboards once were) and was looking to weld new steel in. What gauge steel is recommended, and what do I do with the fuel lines along the side?

I just cut out some of the bad and took it with me to the steel yard. Then did a comparison. Just watch as when you cut a piece, it will be thicker at the cut. Just clean the edge to reveal the exact size.

I think that the factory uses 20 gauge, but I replaced mine with 16 gauge, not a big price difference but it should help a bit on the strenght and gives me a better foundation to weld to later on.

i have a 79 and I used .040" (bewteen 18 and 20 gauge). I needed to form around the tranny hump. Even at .040" this was a challenge but made for a solid floor.