Seats should never (IMO) be bolted just to a sheetmetal floor. As Tech suggested, a 2X2 square tube welded to the frame rails works great. Bolt the seat tracks to this crossmember with holes thru the sheetmetal floor. It's ok to use tabs of at least .250" welded to this crossmember to bolt the seats to. Seat belts should be attached to the same crossmember with .250" (minimum) tabs welded to the same crossmember.

Seats and seat belts bolted to sheetmetal floors are unsafe in an accident. Hard to stay belted in the seat when the bolts rip out of the sheetmetal!!!!!!