FWIW, My car has your standard poly bucket seats and I've found out that they really need some back support to take the strain off the seat itself and the floor mounts. Mine actually cracked the floorboard after a couple years of racing it and the seat has more lean to it now than the passenger side seat.

I wouldn't recommend using them unless you have or are putting a roll bar in the car. Not enough internal strength in the seat\ to support a person.

Mount the seat wherever you feel most comfortable.