My -8 empennage was non pp, so I have a little experience in this.. here’s what I did, you can take it or leave it.
I first marked out where the stiffeners go, then made them to length, tapered them, then drilled them to #42 or 41. Don’t go to final #40 yet. Then I laid them into the skins on their marks and matched them to the skins with #41, clecoed in place. Then I final drilled them to #40. Be careful not to debur too much.. the thin skins are easy to over do it.