It's true that mounting the shoulder belts to the floor can compress the spinal vertebrae in a crash. The angle of pull is all wrong. Shoulder belts should be mounted even with the shoulders to a maximum of 4" below the shoulders. Also, pay attention to the crotch strap mount. DO NOT attach it at the front of the seat. See the attached photo for proper 5-point belt installation.
If you want to use shoulder belts without a roll bar/cross bar, then rig up a mount back at the shelf panel (package tray).