You 'need' to install or 'want' to install. I don't know what state your in, but I registered a LHD 1972 Pontiac in NSW with the American GM belts. They were lap only, as the car is a convertible. I had no problem. I fitted lap/sash belts (non retractable) later on as I use the car as daily transport.
I can't tell you any more than that.