Years ago when I did the Pacific Coast, I also started in Seattle and when I hit Longview, I stayed on the WA side and crossed at Cathlamet on the bridge and little ferry. This time I am heading East. Looking at the traffic cam photos of the bridge, I think I will take the tip from Jeff and take the back route that parallels I-5 on the WA side down to Vancouver, WA and stay on the WA side and head up the Columbia River on Rt 14, crossing to the OR side at Umatilla. I fly out on May 11. My first night will be in Tukwila, a block from an REI where I can pick up fuel for my stove and any last minute incidentals. My first night on the road will be at either Belfair State Park or Twanoh State Park. After that, who knows.
Thanks everyone for the tips and suggestions.