What I did was keep as much original metal as possible and just patched the bad areas. I got partial floor pans and just cut from that what I needed and double lapped the welds. Then remove the rest of the rust the best you can and POR 15 it. That made for a sturdy repair. And it's all under the carpeting anyway and not real obvious looking underneath. Doing whole floor pans can be tricky and you might be sorry if they aren't really good at it.