Do not bolt the shoulder harness to the floor. That could be bad. The 89+ should still have the place to mount the seatbelt on the B-pillar (2nd hand info, i have not done this) and you need to have a bar made that will bolt to each side in those holes and goes all the way across the car. that is safe to attach a harness to.

Although, for saftey reasons, you should NEVER use a 4pt (or 5pt) in a car w/o at least a 4pt roll bar. In a rollover the DOT 3pt harness allows your body to move incase the roof collapses, a 4pt does not.

-Dustin