You'll also want to check the stem for ridges or burrs around the edges of the openings and along the inside edges of the clamping split. If there is any then some sanding would be in order to remove them. Otherwise I'd suggest just use something to lightly wedge apart the clamping split line so you can tighten the top cap more normally. Yes, I totally agree with the others that the cap will move the stem just fine. But it'll affect how much torque you need to do this. And that may lead you to setting the headset up without enough or with too much if you have some difficulty in adjusting to the new setting brought on by the extra torque needed. Wedging the split like is easy and lets you do things the way you already understand. It won't need much and you'll want to use something that is plastic or wood so it's non marring.