The best way to determine the length of the driveshaft is to mount everything into place and measure the distance between the trans and the rear end. There will be a flexible piece on the new driveshaft which will go in and out when the car is driving over bumps and dips. The flexible piece will take care of the difference in driveshaft length.
Take a few pictures of the situation and sent the shop an example of how the driveshaft should look like.