It is possible to fit an xt660x wheel in an xt660r swing arm. You can use the existing spacers and its all good.
A 150 Tourance fits easily and I suspect you could squeeze a 160 in there.
A 160 might just touch the plastic chain guard but a washer or two behind the front tab would sort that out.
Plenty of clearance to the actual chain and swing arm.

__________________
--

XT660R 2006