Weird, I have the same boat and had the same problem. I had to actually cut off 2 bolts as they just spun freely, not tightening nor loosening. So what I did was two-fold. Firstly, I used some fibreglass cloth and liquid and filled in the holes as tightly as I could. Next I drilled 4 new holes in the base of the seat bracket itself and in the floor, making sure these 4 new holes lined up. Once the fibreglass was fully cured I drilled pilot holes and readied the seat bracket to be refastend. Next, I used butterfly or toggle bolts (not stainless though as I couldn't find any) and using really long bolt, fastened the seat bracket to the floor using the new holes. Then I used some new self tapping stainless screws and secured the bracket to the repaired fibreglass holes. All totaled this took me about 2 hours and has held up for the last year. The passenger seat is the one repaired and it's seen its share of heavy hitters sitting in it. Hope this helps.
