I'm a student horn player that plans to play in college. By the time in graduate, I want to invest into getting a good mouthpiece. I am interested in getting an Alexander mouthpiece, but I'm not sure what kind. I need something that would play well with a Conn 8D, and is a mid-range mouthpiece. I was looking at the Alexander 8L, but I'm not exactly sure how it would fit with a Conn 8D. Any suggestions?
It seems to me you're starting from the wrong end - you mention the horn the mouthpiece should go with - but what about the player? Your mouthpiece should be one that suits YOU. You might make some concessions to the instrument (for example, to do with how free-blowing it is) but these come second.
Your best starting point is the advice of a good teacher and a mouthpiece that is not extreme in any respect; stick with it until your playing and your experience allow you to experiment.
On a technical point: I believe Alexander horn leadpipe receivers are configured to the so-called "E" taper, which may mean their mouthpieces will not fit well into the standard receiver.