Do you have a lowered pan for the seat to sit in or is it a flat floor. Check my build log for pictures. In my car the drivers seat is in a lowered pan and the passengers is on the flat floor.
On the flat bottom side: If you run the bolts upward then all you have under the floor is the head and a big diameter washer. Thread the floor. It will retain the bolt somewhat so you can do the seat in/out alone or it will make it a bit easier on the wife to hold the bolt head under the car while you tighten it. Use nylock nuts so it all doesn't fall out the bottom.
In the lowered pan side: Make brackets so that the mounting bolts (bracket to car floor) run horizontally out the sides of the pan. Tight but doable and will save ground clearance.