can't be any worse than this...
notice in the lower left corner of the before picture, cab corner rusted out from cowl vent.
also in the before picture: you can see the heat marks on the firewall seam
where i welded it from the inside under the dash.
then i cut 6'' pieces out and welded it all together, 3 pieces: cowl, inner cowl and firewall
if you have rust through above the seam, that is from water & rust in the cowl vent.
chances are your rusty floor is from the cowl vent too...