IMO It's probably not the distance that is causing the vibration. Most likely the angle. I know a lot of kits come with shims to straighten up the driveline.
Nice I have the bench seat. Mine is my first brand new vehicle ever so I'm scared to cut or drill holes. So I made a removable mount out of a couple 2x4s that wedge under the center console and hold my radio(Ham not CB) out front in a handy location. Pics coming in a few!