I used 36" on my 2004 Eagle. I think 40" would be too long. Take a yardstick and use it to rough measure where the strut will be placed. I think you will find that the end of the struts will be close to the side of the top. Be SURE that the horizontal distance between the upper and lower mounts is more than the length of the strut when collapsed or the roof won't lower completely. You can calculate the required minimum distance with the Pythagorean Theorem where the strut is the hypotenuse, the vertical distance the roof rises as one leg, and the horizontal distance between the mounts is the other leg.