I just want to be sure about this, but the headliner bows for a '66 coupe should simply fit into the holes in the car up to the little nubs that stick out, right? I've been having some issues with the 2nd and 3rd bows from the front slipping and sagging down. The headliner is ~30 years old and deteriorating, but it's still in fair shape. The 3rd bow from the front somehow managed to be jammed into one of the holes past the nubs and won't stay upright. The 2nd bow from the front alone has been frustrating to keep upright, although both ends aren't jammed or anything. There's no trick to keep these things upright is there?

 

With the headliner fully installed you should have zero problems with the bows. The headliner should prevent them from moving at all.