I recently purchased the replacement day-night mirror glass from Paragon ($40) and installed it easily and I'm very happy with the results.
I used a plastic pry tool to go around the edges of the rubber and get the glass to come out. And then reversed it to get it back in.
The stainless backing cleaned up perfect using Nev-R-Dull, and I used some armorall on the rubber before removing the glass to help the rubber given it's 46 years old.
Two words of warning.
(1) Before I dropped the $40 at Paragon I purchased the replacement day-night glass from Zip Corvettes for $27 (I'm cheap!). Well the Zip glass is NOT the correct size (it's about 1/8" taller). My guess is this mirror glass is the one that's used in the reproduction mirrors. Zip is not the only one with this incorrect glass. So if you're going to do the replacement yourself I'd recommend going with Paragon as I know their part fit.
(2) Before going the glass replacment route I had order the reproduction mirror (from Classic Industries). It looked nice, but was a hair taller as I mentioned earlier. The reason I didn't keep it was that the mounting arm was thinner than on my original and it didn't quite stay "still".
Finally, there's someone on ebay who sells restored mirrors for $90 + shipping. At this point I've spent enough on shipping and return shipping that I should have just gone that route.
