About the only thing I can tell you, since I'm not well-versed in Olivetti typewriters, is to turn the machine up on its back and try the shift key, see what moves, and what stops its motion.  The one Olivetti I did see had a long threaded rod under in the middle with jam nuts on both sides of it.  It would be my guess that whichever jam nut set is contacted on the termination of the shift downward would be the ones that need adjustment.  In this case, with the uppercase being too high, is that the basket isn't coming down all the way.  First, see if anything is stopping the shifting from its full travel--it is quite possible something else might be impeding it--especially if the travel is stopped before it hits the jam nuts.  But, if it does hit the jam nuts, then these will have to be adjusted upward until the basket comes down far enough to be in alignment with the lowercase letters.  You'll want to be sure first, that the lowercase letters are typing where you want them to--with the top being the same darkness as the bottom, and vice versa.  If they are, all you have to do is just bring the basket down far enough to where the bottoms of the lowercase and uppercase are touching in the same height as one another.  When you have this, you can tighten down on the jam nuts to hold the adjustment.  Let me know how you come out.