The failure of the windshield gasket (actually, a bead of buytl-rubber sealant) causing interior water leask is vastly overrated. the body of the car has two holes at the lower corners of the windshield to allow and water under the trim to flow out.

that much water in your footwells is likely coming from some cable of electrical harness which passes through the firewall.

Begin looking up around the hood release.

It helps to have the carpet removed and, after flooding the hood hinge area with water, watching for the trickle to begin inside on the floor.

Actually, start the water a bit lower then the hood hinge, down under the brake booster, where that black, triangular panel is sealed and bolted into the firewall. Just possible that sealant could have failed. (but I'll bet it's some cable, wire harness, or hose).

Water and check for leaks. Move higher up to the hinge area to check for any failure of a body seam seal, then up over behind the engine to check for heater hose pass-though leaks. Finally, the windshield and sunroof, if any.

My remedy for rotted floor panels is to cut our the rust, make a paper template of the repair area with a 1" overlap all around, cut a new piece of metal (I LOVE old washing machine panels! Already painted!!!), then lay doen a thick, even band of brush-on seam sealer. Drill and pop-rivet new patch. Coat inside and outside with more seam sealer.

Note: Under the car, I also first remove the undercoating. I have found it helpful to first heat it with my Benz propane torch, then scrape most of it.

Some wire brushing with a knotted wire wheel on a drill, maybe some oven cleaner to remove residue.