How do you measure the shock I need? Lid doesn't currently have one?
You measure center of stud to center of stud to get extended and compressed lengths. So it will depend on your hatch or lid, but measure with lid open where you would mount when it is opened. Then measure for the compressed length.
The SL35 is a gas spring with a 10mm socket. The SL50 bracket has a 10mm ball, so they will work with each other. The gas spring will not work with the SL50 blade. Be sure you select a SL50 bracket with a 10mm ball.