my 1999 f-150 floor is totally rusted

I would just get some sheet metal and weld it in. I had a Volvo that rusted out before I could catch it, and I just welded in some sheet metal and it worked until the car had 336,000 when I sold it, and it was still driving. Or, you could go to a junk yard and cut out some old floor boards but I think that would be a lot more work.