A plug weld is when a hole in the top piece is filled with weld material thereby attaching the top piece to the bottom or supporting piece.
Remove your old floor pan, leaving enough original material around the edges to support the new floor pan. Drill holes every few inches around the new floor pan so that these holes will be over the supporting lip of the old floor. Weld through the hole in the new pan to the supporting material. This is a plug weld. The size of hole depends on the thickness of the new material. Use some scrap and practice. I would start with a 1/4 inch hole about every 2 inches and see how that works for your situation.
