My clymer manual shows 1.5-2.5 ft-lb of torque.
FWIW these torque values aren't very helpful in actually building a strong wire spoke wheel. Proper, uniform tension are the key factors in building or maintaining a strong and reliable wheel. It's really easy to accurately test for tension in a bicycle wheel because of the finer gauge of wire, less so with these big bulky motorcycle spokes (at least with the tools I use :-p)
With a motorcycle wheel... if you're getting a nice musical *ping* out of the spoke, and not a dull thud, you've probably reached sufficient tension. If the wheel is true and round (1-2/100" should be sufficient) and none of the spokes ring at a tone more than a full note apart (you can use a cheap chromatic tuner, a guitar, etc. to gauge this) then you should have reasonably uniform tension across your wheel. In a perfect world anyways. Sometimes you just get a spoke that won't fall into line with the rest. Usually time and stress-relieving will help to sort them out to some extent.