The watershed area for Stony Fork is pretty small; you could gage the larger streams in the area, but probably not get a real picture of Stony Fork. The organizers will post the level of the stream on Friday at registration, and give an update on Sat if it has risen to difficult levels. If it gets too high, they will cancel the stage.
Rather than spend time on the water levels, I would do decent prep for the crossing as outlined in several posts, and then go worry about other things! The organizers will keep you up to date. I would worry more about a vary rainy Saturday turning the stages to goo, and seeing if I could procure a good set of mud tires, much more than the level of the water crossing. Muddy roads will be much more of a challenge to completing the event than the water crossing.
Regards, and best of luck!
Mark B.