How much should it be shortened....

I would call your drivesgaft shop and ask them what exact measurements you need from them.
Different shops ask you for different measurements.

My local shop asked me to measure the very rear surface of the output shaft splines to the flat mating surface of the rear yoke which would be the center of the rear U-joint.
This measurement should be made with the car on the ground.

You should have at least 1.5" to 2 " of forward travel of the front yoke while installed.
This will assure you that when the pinion angle changes under load, the driveshaft has some room to travel forward.
Too much travel and you are gonna jam up your transmission guts or the thrust bearing on the engine.