Mark, tempered glass can not be cut, period. 99% of all curved glass is tempered, windshields being part of that 1% that is ALWAYS "Safety plate" (the two layers of glass with plastic in the middle). I am not certain of the actual process of manufacturing tempered glass. But as the name implys after the glass is formed into shape it is "tempered" with heat to have a "layer" of super hard glass on the outside. When that super hard layer is penatrated, the ENTIRE sheet "fails" and the ENTIRE sheet falls apart into tiny 1/4" sized pieces.
It is strange stuff, you can throw a brick at one of these windows and it MAY bounce right off! OR, you can take a tiny piece of ceramic from a spark plug and throw it at it and it will break into a million 1/4" pieces! That is a little trick if you ever have one to break, a tiny piece of spark plug ceramic WILL break it almost every time!
The 56 Buick (as in all GM cars) had a safety plate windshield and side glass with a tempered back glass. The side glass is all perfectly flat by the way. There is no car made today that has this, all glass but the windshield is tempered on 99% of cars today. There are a few exceptions in high end cars and cars with side air bags (a very few) that have curved safety plate on the sides.
I did a little Mythbuster experiment on sandblasting tempered glass on this thread. Urban Myth?
It can be ground with a wet belt. I know a glass guy who converts 65 Mustang Coupe door glass to Convertible glass by grinding the rear corner down a little. But it literally takes DAYS to do it, grinding a tiny amount every day.
Nope, cutting the curved tempered glass has been the "Holy Grail" top choppers have been searching for since the first car with it was chopped.