i'd hit the coolant line right off the bottom of the motor, passes almost right under the recoil on the exhaust side. pull the can, look through the frame and you can't miss it. That's right off the motor and will provide the most accurate temp of the water FROM THE MOTOR. If you want the water temp going into the motor, hit the one hose before it goes into the head.
Ok, I have an M7...D'oh! can't say about the M8 but they can't be THAT different.....Sorry!!