It has been soooooooo long since glass has been in my car I honestly cannot remember exactly but I recall the headliner having to be pulled down for screw access at the top both front and rear and the corner triangle shaped pieces have a stud on them that bolts after the glass goes in. I think those pieces have to go in when the windshield is roped in. I am pretty sure the Impala is different then the other non-Impala models. Glass was installed at Fisher Body before the car was delivered to Chevrolet for final assembly so I do not believe it is in the AIM and I have yet to find a 1958 Fisher Body Assembly Manual.
The front glass has a two piece molding on the top that has (6) post style clips that mount them to the top of the roof line before the headliner gets installed. Make sure you use some sealing compound (strip-calk type by 3M #08578) on the stems to seal the molding clips at the stems/outside roof mounting holes (using speed nuts to secure these) before installing the headliner. The lower front molding gets installed into the glass seal prior to installation if I remember correctly. Other's are correct that the rear glass moldings get pushed into the clips that get installed around the rear glass opening pinch weld "prior" to installing the glass. Just make sure to install one on each half (on each side of the union) of the lower molding to hold them in place when you install the two lower half's of the molding. The lower union is the starting point to install this molding and work it around and up to the corners. The one piece upper rear molding goes on next then the two corner pieces (which should get sealed around the stems like the front upper moldings did). Hope this was helpful.
That is correct, only on the molding posts to seal the moldings at the top roof line and on the side/corner trim pieces (front and rear). There is another 3M product that can be used in the windshield/back glass seals. It's called Bedding and Glazing Compound #08509. This product should be put into both the front and rear seal grooves prior to installing the seal onto the windshield/back glass pieces. It doesn't get hard and shrink up which helps to keep water out between the seal and the glass units after installing them into the car.