The 1971 K10 Suburban I parted out years ago had a factory marine plywood floor from the rear seat area rearward...it was the strongest part of the truck too,all the rest of the sheet metal had turned to rotted flakes ,but that plywood was like new,it was impregnated with fiberglass resin or something similar..
I was able to kick the rear quarters off with a few kicks on that truck it was so badly decayed,but the wood floor I had to cut with a sawsall to get the rest of the body off..
If your unable to buy the "right" floor repair panels,you might be able to use pickup floor panels from a boneyard,or some other kind of flat sheet metal..I've used everything from filing cabinets,metal roofing, to water heater "skins" to patch up rotted floors and quarters in the past...even old washing machine or dryer housings !..the porcelain paint resists rust well...galvanived steel wont rust ,but welding it can make you real sick unless your careful to do it outside with a fan going and avoid inhaling any of the white smoke it makes when heated...