Never done one Jonathan, but probably will do the 65 myself. 

The key is to get the glass installed at the correct position (stick out) relative to the channel, in that it has to be just right, or you will have loose trim, or you won't be able to get the trim clicked into the clips.

It need to be carefully mocked up using the correct thickness of rubber spacers before committing to the sealer. There is a never ending discussion on line about what sealer is best, lots of you tube vids as well. I'm on the fence as well for sealer.

All the clips are available. Including the slide in clips on the lower reveal molding. I've seen them all on line. Same as Impala I suspect as it used the same cowl. Not hidden wipers like the 67 USA Pontiac.

I'll try to dig up some of the likes to the above. Hopefully the more experienced members will chime in on techniques.

While Butyl tape certainly seems easier and probably cleaner, it probably isn't so forgiving in sealing uneven surfaces. The tape would have to be at least the thickness of your spacers. Unless by chance you could find tape the exact thickness that you could ditch the spacers. But then you have a potential situation of the glass subsiding on the tape.

Urethane while filling in the imperfections in the channel, and perhaps completing a better seal, can be super messy and will tend to push out into view on the glass from the outside and inside the car. Probably need to incorporate dam tape with the Urethane.

Maybe a hybrid, spacers to set the height, Butyl tape, then Urethane around the entire edge of the glass for a sure seal across the spacers?

That is a big piece of glass Ray!

Mike, Good point on using a "dam" with the urethane.  i just did that with the side glass on my wagon,  really worked nice.

As for Johnathan situation, if you are not experienced with glass installs, then i would highly recommend just going with a 3/8 square butyl from any autobody supply store.  also get a small bottle of pinchweld primer to go on the chanel.  You will have good luck with this system and it is very easy and forgiving for a beginner.

Also, if you use butyl, you will be able to remove the glass at a later date if you ever need to.

Urethane is better quality, but i have seen both urethane and butyl "let go" over time.  Urethane is a total bitch to cut out at a later date, you will be fighting it for hours with sharp blades.  just concider that before choosing. and without the "dam"  you will make a real mess of the install.

Professional glass installers are prohibited from using butyl, but the rest of us can.

Also just a side note, on a classic car with window mouldings that snap on after the glass is in.  These mouldings were designed to also retain the glass in a crash, unlike modern cars with no mouldings, when the seal lets go the glass just flies out.

Also,  the old debate on butyl or urethane.  our old 60's cars never had true butyl installs from the factory, it was a bit of a hybrid formula that let go in  short period of time, so alot of these cars got resealed with butyl early on in life before urethane became the norm.

3/8 will be the proper thickness, you will have to push down slightly to smoosh the butyl down as you are installing the trim

You use the spacer blocks on the bottom to space teh glass centered in the frame.  the blocks will go along the bottom to space it away from the clips, also to center it side-to side.

the glass clips should be available from a Chevy parts supplier. Try Antique Chev in Dundas . Sometimes the corner clips are not in the kit and must be ordered separately

Dale @  NOS