I usually cut on the inside, using a sharp blade, less risk to the glass. Just cut off the lip inside the truck, the glass will push out, have someone catch it as sovagary said.

My \'64\'s in the Gallery
Pictures in my Photobucket
1964 C10 Custom Cab 350/700R4
1964 Suburban 350/700R4
1979 Ford F350 4x4 400/c6