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.