Back in PA I used to fish the Delaware and Lehigh Rivers for smallmouth bass. These are both large rivers (300+ feet across) and I did best in areas where islands broke the river down into smaller "trout stream" proportions. Here I would fish a large Stimulator dry fly with a 24" dropper down to a #8 or #10 Prince Nymph. Both of these rivers had excellent stonefly populations and those river smallies gobbled that nymph like crazy (mostly chubs and sunfish on the Stimulator). At first light in June/July I could get them on a Madam X dry fly, but the Prince Nymph was more consistent by far.
If you're really interested in nymphing for smallies, go right to the master himself - Harry Murray. His book is the best I've seen on this subject (illustrated by Dave Whitlock) with large portions dedicated to his patterns and nymph fishing techniques.