My 06 M2 from the drivers seat seemed to be pretty quiet and vibration free. But my wife said when you went in the bathroom the noise at speed was pretty loud since the tires and axle where just below.
I am thinking your situation is pretty normal.
I must mention that my sissy pusher on a FL chassis with 400 cummins is insanely quiet and the ride blows away both conversions. The only drawback so far are cross winds over 20 mph. Oh and that WOW factor you to get.
I am actually considering building a largecar, something like what CSM is doing. For nothing but fun, maybe pickup a truck in a year or so. I had alot of fun redoing the interior on the FLD and would love to find a older Kdub or Pete and trick it out.