I got really bad floor pan rot on the passenger side rear seat looks really bad like holy jeezz last year it wasn't to bad but now it's like a big giant hole I placed a piece of sheet medal over the top of the rot an screwed it down into the floor pan I plan on putting a piece underneath also is it alright to weld it into the exsistening floor or is things gonna catch fire or should I just replace the entire floor pan

the only proper way IS to weld it...as long as you move any wires or lines going through the area you weld nothings gonna catch fire, and obviously the carpet has to come out, and anything else in the way.

Like Project said, only proper way is to weld the floor. Also, the only proper way to fix the rust is to cut out the bad sections completely and then weld in new sections of sheet metal. If you don't get rid of the rust now and just weld pieces of sheet metal over the bad parts already, your not truely fixing the issue. Do it right the first time.