Steering head lock-ring tool
Moto Guzzi V700, V7 Special, Ambassador, 850 GT, 850 GT California, Eldorado, and 850 California Police models
I extracted this recommendation from Ralf Brinkmann off of the old Yahoo! Loopframe_Guzzi news group (which has now moved to Groups.io).
Here are a few places where you can purchase the special hook wrenches that will permit you to easily tighten and loosen the two steering head stem nuts. You'll need two wrenches. The outside diameter of the steering head nuts is 38 mm.
Thanks to Benjamin Mauceri for sending me the following information in a private e-mail:
The fork collar tool fits the steering stem nuts perfectly.