I realize I'm a bit late to be answering this but just in case others come to find answers on this as well...
I run a 50/34 chainring combo and usually an 11-32 cassette for the PNW gravel stuff. While I haven't done Leavenworth, I have done Winthrop, Ellensburg, and Ephrata. Ephrata's the only one I feel I can get away with a 36 small ring and 28 large cog on. I'm speaking from the perspective of being a pretty strong (but also fairly large at nearly 200lbs) local Cat 1 and Open/Expert.
The reason for this is that there are quite a lot of steep bits on the gravel and you can't stand or you'll spin out. So for comfortable (relatively speaking) seated climbing on this stuff I find I need at least a 34x32 (or choose your own ~1:1 gear).
As for tire choices, those look solid. Leavenworth and Winthrop are known to have pretty rough descents. Ellensburg a bit too (flatted my 28's more than once on the last descent there a few years ago and now run at least 35+s when on the gravels). It's getting easier and easier for me to admit that I like big tires! (Que Sir Mixalot)