The first floors I worked with required me to remove the old seat supports and weld them to the new pans. I can't count how many welds were drilled out before using air tools to pry both seat supports from the original floor. It sounds like this may be your only option.

My new full floor had everything attached and made the installation process so much easier.

I am never surprised with the lack of effort put in when people who should not be working on cars do work on them. My current project had 3 quarter panels welded on each side. Instead of replacing the original they continued to weld one on top of the other.

Good luck with your project