grab a small propane torch, makes removing glass that much easier. and the proper knife tool(I have no idea where to get one), suction cups are a must for handling/setting the glass into place.
Heat up the urethane, cut the urethane from the inside of the car(you can use a regular box cutter but those are dangerous when they break). ALWAYS start at the top, and have a couple wood wedges to stick in the top corners to make the sides/bottom easier to cut.
after you get the glass out, make sure the urethane that is left has no flaps or anything, try make the old urethane as smooth as possible, v notch the new urethane and put a 1/2" tall bead on the old urethane. Place glass and tape the top to the roof, crack a window 2" and NEVER slam the door(air pressure from slamming a door will pop the glass up).
Typically I cut them out with thin wire.. But thats not mustang thats out of the million slammed s10s ive owned. My mustangs windshield was super caved in when I acquired it and I went around the edges to heat it up nicely w a torch and then used a wire to "cut" the adhesive between the windsheild and the body.. It was a bitch..but got it out and got a HUGE discount when Ppg or whoever it was came and slapped a new one in...