Trimming a curved windshield requires a delicate hand. The best way is to use a belt sander (not the hand held kind) that has a coolent flood. Most glass shops will have this piece of equipment. It'll take several abrasive belts because it needs to be sharp, lots of coolent to keep it cool and patience. You'll need to make blocks to support the windshield while working it, especially when you work the corners. Any vibration and it'll crack. Water jet can also be used, there are some services that do this but I'd be concerned about the process with big curved glass.
Is it a big window cab? Rear window glass is more difficult if it's tempered.
I've also seen glass cut down with a sand blaster, but it needs to be supported or it will crack. You'll also want to use sand blast sign tape to protect the glass.You'll want to finish the edges with a belt sander though. Remember, heat and vibration are your enemy.