You want to put your level on the bottoms of the fore and aft spar carry through tubes. You are right, a 4 foot level is too long, but here are a couple things you can do.
-Take out the turtle deck plexiglass and let the level go out the back.
-Put some spacers between the level and spar carry through tubes so the level is lower (I used a couple of pieces of 2x4).
Looking at the manual it appears there should be a gap under the aft spar, not the front one. BUT . . . the main thing you want to check it that the rear spar is 1/2" higher at the 159" station then the 3" station.