It is doable, but means a bit of tweaking ...
You will need some longer M4 fastners (at least 15mm longer) and 4, 12 to 15 mm long spacers, with a 4.5 or 5mm hole.
As mentioned, the mounts are 90 degrees different, but the 346 mount will fit in.
You will also need the fly-lead to attach to the 346 power lead to the OEM GPS socket for switchable power.
I am planning on ditching the Nav 6 on my bike and using my 346 instead - on the whole, a simpler and better device.
If I'd got the hardware, I would have done this this weekend and photo's to show, but not got fastners suitable to attach the 346 mount with.
Popping into B&Q on the way home for hardware ....