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