I'm the guy who did it and I'm nuts.
But seriously take the bus down 99w to the end (Tualatin or Sherwood). Then bike down 99w which isn't too bad, there's some great scenery. The trouble is when you get to Newberg.
Take Rt 219 south across the Willamette. But be absolutely sure to take the entire lane while crossing the bridge. It sounds selfish, but if you go to the shoulder, you'll find the shoulder ends and the steady stream of cars will be impossible and dangerous to share with. I've brought this up with ODOT who doesn't care. There is also a ferry which is a safer way to cross, but I don't know the best way to get to it.
Use the state bike map (get one at BTA) to wind your way down the farm roads through Salem and down along the river.
If you have time, I can look at the bike map and tell you which roads.