One everything is ok, remove the pins, or slide the horizontal stab out, and epoxy the horizontal stab in place. Recheck the distance to the main wing tips with the string and adjust as needed.
Let dry.
Are you assuming that the space for the horizontal stab is now perfectly sanded to hold that stab level? My problem was that there was too much space there and the stab would hang lower on one side. I tried taping it to the vertical stab but then it wouldn't stay centered. The tension of the tape would slowly move the stab to one side. I couldn't get it where it wouldn't move.