I bolted my Zumo 550 motorcycle holder directly to the dash. I had to reposition it about 1/4" lower because the swinging locking bar was blocked -- too close to the top.
I recently upgraded to the Zumo 660, and the holder uses the same bolt pattern as the 550 mount. The 660 mount is much slimmer, and even though the screen is much bigger, the footprint on the dash is actually smaller than the 550.
The locking system is also different, using a push button on the right side to release the unit. Mounted directly, flat on the dash, there isn't enough room for my finger to release the GPS from the mount. It needs about a 1/2" spacer on the top bolts to allow room to depress the release button.