I used 3.5 inch between centers. Upper slots are 2 inch radius with a little extra depth. It worked perfectly, even for 12 ga side-by-sides.
I have a thread here somewhere with WIP pics etc...
Here's a pic of the upper cab assy I built for a friend. I traced out the bottom/butt on a few different styles of guns then built a guide template and used it to rout out the bottom holes. A good fit ensures the guns won't move much. I played around with tilt angles and upper slot heights to make sure each firearm is stable.
Here it is the finished 2-piece cab "loaded" at it's new home. The pic quality wasn't great but you can make out the spacing okay.
I really enjoyed making the gun cab a lot. It was interesting and I got to work in a few areas that were new for me.
Chuck