Go to Auto body specialties. I just put a set of front floor pans in my 65 Dart. They called me right after I order them on-line. They said they would almost drop right in after they arm trim to the size I want. They did just that. Excellent to buy them from.
I used 18ga. Sheet metal that i formed over a 2x4 and used self tapping screws to pull it in place. It came out great, use rubber undercoating on the bottom. And cover the floor with insulation material. 24" sheets work great.