I was thinking of a Flowmaster 70 series (nice big case for sound, but plenty of flow) and a nice mandrel-bent 2.25" or 2.5" system from the manifold, and possibly a high-flow converter. With a cam and intake upgrade, I would think the slight step-up in flow would maintain good torque.
By the way, those of you with turndown before the axle, do you get fumes building up under the cab when there's no wind?