i have always installed 3/4" pressure treated plywood over the entire van cargo floor area before installing our truck mounts (have done this 5 times over the past 30 years).
First I use 40 lb.roofing felt to make a pattern to cut the plywood. Then I keep the felt on the floor, and use it as a gasket between the metal and the plywood, screwing the plywood down with stainless screws just long enough to penetrate 1/2" into the metal van floor (pre-drill holes).
Finally I install a diamond plate design industrial floor matting instead of carpet (glue down). Carpet will not withstand abuse like the rubber floor matting will.
It will cost a lot more to do the metal thing by the time you pay someone to weld it for you, and you'll still have metal which can rust again.