We travel that route quite frequently. Snoqualmie is 4 lanes headed up from the west and the slower vehicles are well behaved and stay to the right. I believe it is 4 lanes from the east as well...can't fully recall right now. But it is definitely at least 3 lanes.
Blewitt Pass, Hwy 97, is a NICE highway, though it is mostly 2 lanes. There is a passing lane for the last few miles in each direction. I have gotten stuck behind slower vehicles but never where there is a passing lane. It's not as steep and windy from the south (west).
This route is definitely my favorite when headed to that area. The only trouble is there are basically 3 passes when traveling this way if you take Tiger Mountain (passing lanes in each direction.) This can take a toll on your mileage, but when towing you should expect a hit compared to not towing.