The original seat supports are spot welded in and do perform a measure of stiffening to the floor pan.
You could cut the majority of them out using an angle grinder.
Make sure you protect the interior from sparks, they will burn holes in anything
I would suggest grinding off the seat support spot welds with either a small stone in a die grinder (maximum control, minimum speed) or angle grinder (minimum control, maximum speed)
You only need to grind down to the bottom of the weld depression and then using a cold chisel or multi grips you should be able to tear the sheets apart
Try to avoid going through the floor pan, you will take in water if you do
Once you have the old panels out, grind off any weld dags to give a nice smooth pan. Treat the bare metal with a degreaser/metal conditioner and once squeaky clean, spot prime with etch primer.
Follow up with a squirt of primer and a top coat. Carpets and underlay are notorious sponges so make sure you don't invite the dreaded tin worm to your floor pan
Ditto with your new mounts, make sure they are protected from corrosion.