until you have a bar mounted across directly behind your shoulders I wouldnt get a 5 point, mounting the back peices to the floor is a BAD BAD idea... very bad. You need the shoulder mounts right behind your shoulders so when you need to stay in the seat the pressure of going forward dont yank you down toward the bottom of the seat.

 

get a roll cage with a middle bar and you would be set, a 4 point would work fine.