I think the difference in space is negligible.
You are right that they dump straight out. On the night figther version they had guides/deflectors to keep them going down as they ejected to keep them out of the radar or prop arc or something. You don't see it as much on the day fighters. On the photo there, the left gun's outboard side is the ejection chute. I imagine, based on exit chutes, that the right gun is reversed so that both feed from the center and eject to the outside.
I have seen that photo and others as well. Interesting to note: The mounting rails for both guns are actually side by side. The one gun is just slid back further on the mounting rail. Had the LW came up with a different loading system, the guns could have been easily placed parallel (instead of staggered)