Don't over think this. If there is no slot in the horizontal stab for the long piece to fit into then it was meant to be glued flat on top of the stab with the two gussets on each side keeping the rudder in line with the wings and stab and giving it some strength. You don't want to compromise the size of the cut out for the elevator deflection so glue the two pieces so they are flat on the bottom. Then sand the leading edge of both pieces until they blend into one straight line. Shape the rest of the vertical fin as per the drawings and be done with it. 1/4 inch difference than the plan will not make one bit of difference in how it fly's.

Dennis