I have recently done this.

I approached in in two ways. My seats mounts were rotten. Right through. So I cut a corresponding piece of 18 gauge and slipped it in the hole from the top. Tacked it on then welded it in nicely from the top.

I drilled some small holes (3/16) in the new metal.

Jacked the car up and welded the bottom of the new pieces and ground them as flat as possible. Then I added another skin of 18 gauge from the bottom. Went back up top and welded the drilled holes up. Therefore mating the top and bottom plates as a laminate.

Painted, undersealed then painted again.

Understand that in a crash your seat will be taking a bit of the hit with your ass in it. the seat belt is attached to the seat. If the seat is not attached neither is your seatbelt.

Overbuild this fix but make sure you grind it flat inside so that the seat will mate nicely to the floor. Paint it, seal it, paint is again top and bottom to make sure there is no rot and to ensure ant seat movement with not chafe the metal and allow moisture in. Fix it ONCE.