you can mount the lap belts in the original position where holden mounted them, as for the shoulder belts, remove the bottom rear seat and mount them along the same line as the rear seatbelt mounts. you will need to drill holes for them and you will need to make backing plates to reinforce the bolts holes where they go throught the floor pan. when you have finished mounting the belts, reinstall the rear seat with the belts coming out between the bottom und the upright seat back where the other belts come out etc.
we can't have multipoint harnesses overhear unlessthe vehicle has a competition log book etc and i'm sure they need to be certified so i'm sure you will need to investigate that part also. but yeah, you'd be better of modding the car first
The best way to answer this is to go & see the rego authorities in your area. When we have fitted either belts or a harness into a vehicle we have had to get the TAC committee to approve all work. This is in a workshop with a lot of experience in modifying vehicles.
Incorrect positioning of the harness mount points can cause the harness to fail in an impact. Add to that the fact that normal passenger car seats can fail if the load in an impact is too great which also makes the harness pointless.
I'd go with Airstrike on this one & spend my money elsewhere until the need for a better restrainment device is warranted.
EDIT: Mounting the shoulder straps to the floor will not be sufficient as the excess belt length will not support the upper body & will cause seat failure. They should only be mounted to a point about 2" below shoulder height.