We mounted mecanums on the kitbot frame, without the kit mentioned above. Distance between the center of the wheels is about 19". We used second stage planetary gearboxes. To mount the gearbox, you will need a plate w/ three holes (for the gearbox) that extends out and mounts to a side of the frame. We used two holes on each side of the plate.
I have attached a two pictures:
http://imgur.com/a/1X2if
and a video of it driving:
https://www.youtube.com/watch?v=uQbVDzJR3bc
Also, a couple of notes on our drive:
There's a ~40lb steel block mounted in the middle to simulate the weight of the actual robot. We started with first stage planetaries, but they didn't generate enough torque to strafe. We switched over to second stage and it strafes fine.