How much should it be shortened....

Have you already ditched the ball and trunnion drive line or are you planning on a hybrid drive shaft? If the shop doesn't know what you want when you tell them the situation, find another shop. All I have to tell our local drive line shop is "I am swapping an 8 1/4" into a '65 Valiant" and they know what you want. I once towed a car to their shop and drove it home an hour later (440/727 and 8 3/4" into a Feather Duster).