I havent done a Impala yet but I did do one in a 67 Chevelle. stretch and pull to the sides . use spray glue but not to much, it will come through the materal. the side trim will hold it in place. I ve got to do my 65 impala wagon next.
The tack strips are secured to the underside of the roof (front and rear)with pan head screws. They were originally a pressed cardboard of some type painted silver. Some of it may be gone, so I replaced it with cut pieces of paint stirrers! The strips above the sail panels are approx. half way down from roof towards the package tray.
Attach with 1/4" staples in a staple gun, factory also recommends gluing just as the rest of the headliner with contact cement first. And yes, the tack strips are accessible under the chrome trim without removing the glass. By the way, the 1/4" staples will probably be too long to use in the tack strips above the rear sail panels, so use shorter if you can find them.