I am planning to try this out when I get into my spare 650 motor soon.
(of course, I have not had much luck this last 18months getting to any of the "Soon" things I have planned, so have lots of salt ready. *grin*)
As to the water pump, the 650 impeller is taller by about 3/8" and the wp cover is deeper, I think the motor rear cover is the same though.
I'll try and remember to check that too.
As far as the auto tensioner, I am pretty sure they only break if you let the chain get loose and "beat on" the guides. if you change your cam chain every 40-45k miles you should be fine.