Andy- I'm on my second season racing 'cross on a SS, last year I geared it 42-20, that was okay on all but the flattest courses. This year I replaced the (cheap, crappy, destroyed by mud) 20T freewheel with a White Bros. eno dos 17/19 freewheel, and started the season geared 42-20. After a severely vertical course (Rainier) I swapped the chainring for a 39T in time for a second hilly course (Sherwood). You found me at the end of our race laying on the ground gasping for breath, but I never actually puked, so that must have been about the right gearing.
I talked to some of the fast SS guys, and they are mostly running pretty close to 2:1, with 39-19 being a pretty common option. I will probably move the chain in to the 17T cog on the freewheel for Hillsboro this week, but I think the 39T chainwheel is going to stay.
46-18 sounds suicidal. Call or email Eric at Sellwood Cycles, he has some chainrings in stock, and is the local expert on 'cross.