You might check http://rideoregonride.com/

Depending on how far east you are riding from PDX, there should not be a lot of interstate on the Oregon side. It has been awhile since I've ridden on the old highway, and I know they have opened more of it since then. I believe the above link will give yo a pretty good idea.

Highway 14 on the Washington side has a good shoulder, and is OK. I-84 and Hwy 14 are the only roads through the Gorge so all east/west traffic funnels through them. I think you will find the ride will be beautiful and scenic, but parts of the route will not be tranquil. I believe that you have done parts of the Pacific Coast. The traffic conditions feel much the same on Hwy 14 as the Coast Route.

IMO it is worth doing.

P.S.
If you plan on getting off the the highway on the north side of the river for lunch or bathroom stops, find out what poison oak looks like.