They did the two different lengths to counter the engines angle the way it's mounted in the car. If you get two new vavles from Nissan, you shouldn't have any problems. You can get the number # 2 valves for better flow. The 1.8 was in the earlies and the 2.0 was in the later. And somewhere they used both for front and the rear but I can't remember which order.
Dave
Dave