I agree with Chris, My 3B hasn't flown yet and obviously has no cracks, I did the SB anyway. I included the forward doubler and used solid rivets, to gain bucking access it is best to remove the tip rib and de-rivet enough of the rear spar bottom skin to get a little more working space.
Straight forward enough, but do realize that the Van's supplied SB kit for the 3 is just raw materials, you will need to cut and bend the hinges, fit and shape the aft doubler. Accurate match drilling and hinge alignment up down / in out is obviously crucial. Work slowly and carefully, I estimate 30-40 hours from initial drilling to final paint fix up for me. YMMV.
Not doing the internal fwd doubler and using Cherry rivets would be pretty straight forward.
As Chris suggested I think there could be a chance doing more harm than good. Chances are if your empenage was built accurately with perfect elevator hinge alignment it may never crack.