40NM is about 28lb-ft of torque. Sheldon is right of course (big surprise); it will be close to impossible to overtorque these with the typical 8mm Allen wrench which is about 5" long.You would have to be capable of putting out 80 lbs of force with one hand, on a short, thin allen wrench. Superman-maybe-the rest of us, no way. Now, if you have allen bits for a socket wrench(-7"-8" long) you would still have to put out close to 60 lbs of force-a lot for one hand.
Put them on with antiseize or grease.Clean the threads on the crankarms very well first.
Luck,
Charlie