the 3/4 would be fine because you are talking about psi. So you are going to have the same amount of air pressure with either size line. The air hose you use is probably only 3/8's anyways. Can you put a storage tank in the shop area to hold air closer to the work area. Make sure that underground line don't freeze up on ya in the winter time. You might have to put it in the ground like 3 or 4 feet.