Hold the elevators up against the rear of the stab and make a mark on the stab trailing edge where the inside end of each elevator is. That will give you two marks near the center of the stab about 3/4 or 1" apart. As long as the rear of the fuselage is in between the marks, your elevators will work.

Dry fit it first and then when you are happy with the fit, use 30 Minute Epoxy.

Jim