Yes, the OEM seatbelts will hold you in the car.
5 point harnesses are not a good idea without a full race type (cross braced) roll bar, as they prevent you from twisting and ducking down in case or a roll over.
Scroth has a street legal 4 point harness setup, that has a weak point in one harness that allows you to twist and get down further. I don't know if you can mount them in a Z. I know my E46 M3 had the rear mounts installed from the factory.
'95 Alpine M3 LTW
'00 Dakar M Roadster
'02 Topaz M3
Red/White SRF #4 (Chassis 561)