Well that's dome.

The Impala didn't come with a dome light, it had gone missing some time in the past 19 years.  The Caprice did have one, so I thought I would swipe it for the SS.  Much to my chagrin, the Caprice and SS dome lights are different, so the the dome light couldn't be mounted.  The problem is the two round holes seen here.  The Caprice dome light screws just thread into the sheetmetal, but the SS has holes stamped out there.

The solution is a super simple one.  I just bent up some sheetmetal to hook over the top of the roof crossbar.

With that there, the Caprice dome screwed right on.

With that taken care of, I could put the headliner back in.  Once it was up, I carefully cut all the holes for the dome light, coat hooks, etc.  Eventually, I might recover the visors in matching suede, but it's good enough for now.