I've put replacement pans in and I've put flat 18 gauge sheet metal in, and I've put 20 gauge sheet metal in. Once its all welded in, the edges sealed, a coat of paint and carpet or rubber mat installed, it makes little difference if flat sheet or a formed pan is used.
20 gauge is easier to work with then 18 gauge is, and if your rubber mat has the jute backing, it will rust out just as fast, its the jute backing that causes the problems. For the record, 98% of all floor pans rust out from the inside. The jute backing gets wet, stays wet for weeks at a time, and rots out the floor pan. Poor window seals, windows left open, poor door seals, holes in the the firewall, vents, or cowl, or snow & mud covered wet shoes & boots all cause the backing to get wet. Gene
