You have adjusted the steering box correctly, so you may well have achieved the best setting/performance. It's difficult to be certain without seeing the car. I assume that the drop arm is a tight fit on the splines and that all four drag link and track rod ends have no play. New ones are available on ebay for about £30 + P&P. The only other area that can wear is the bronze bush in the steering box. You can test this by disconnecting the drag link from the drop arm and pulling the drop arm to feel any play in the output shaft. Rebuilding is about £500, so it's a last resort.
Tony
