Also, I wouldn't be scared of 18ga sheet metal unless you're concern is about burning through while welding. A LOT of the aftermarket panels are 18ga. The orginal sheet metal was around the same thickness. It sure as hell wasn't 16ga!
DS and PS floor pans on my '82 were completely rotted through. I replaced them with aftermarket pans from Cool Cruisers. They were a perfect fit, 16 gauge steel instead of the stock 18 gauge. I consider them an upgrade over stock.