I have spent a fair amount of time riding on the roads on the northern end of the Shenandoah Valley. I enjoy route 42 (and it's northern "extension" = Back Road which continues north from where Route 42 turns east toward Woodstock in the northern Valley).
I enjoy riding on Route 11. Some of my riding friends don't like Route 11 and feel it has a too much traffic for them. I personally have never really had any issues on Rt. 11. It has a center turning on almost its entire lenght and I have found motorist tend to give ample room. I particulary enjoy peddling throug the many small towns on Route 11 toward the nothern end of the Valley (Strasburg, Edinburg, Woodstock, Mount Jackson, etc.)....highly recommend Sal's Italian Restuarant on Rt. 11 in Edinburg.
I do not like Route 340 due to it being more narrow and less room for cars to share the road.
My favorite loop in the northern valley starts in Strasburg, south on Route 11 to Edinburg (could take Back Road as an alternate). Turn east at Edinburg on the Edinburg Gap Road, go over the moutain (a moderately difficult climb) into the Fort Valley. Go North on the Fort Valley Road down through the Fort Valley (several good camping spots toward the northern end of the Fort Valley in the national forest). Once you go all the way down the Fort Valley, you can turn west on 55 and return to Strasburg....about 50 miles all together. The Fort Valley Road stretch of this ride is one my favorite stretches.