i just recently did one ......i replaced my dash so i had to pull the windsheild first.
this was my second attempt at installing a windsheild and first sucessful time.
the first time i cracked the glass on a 71 dart and the mistake was that i tried to re-use the old gasket....old gaskets are not as flexible as new ones and they dont seal once removed like a new one will......getting a new gasket saves a lot of headache unless you find an original gasket that is really nice and not dry rotted at all .....that is rare where i live bcoz its desert here so most older gaskets are cooked and new ones are soooo flexible and seal up very nice.
the other mistake i made on my first attemp was that i seated the window on the bottom first and tried to work the sides and top in ....didnt work
the second and successful attemp i wedged the top of the glass in first and worked the sides and bottom in ....and it went right in.
i also had rope in the gasket pinchweld part on the sides and bottom only and pulled the rope from inside the car ....that worked great and i bought a little tool from a paint shop used for windows .....its just a plastic stick with a rounded wedge end ....worked great and its plastic so it wont tear the gasket.
installing a window sure is a workout and takes patience ....i had some time into it ....but it was worth it to keep the glass in one piece.