I have the Flowmaster Dual kit on my Tahoe. it drops the tailpipes at an angle behind the rear wheels. I would imagine you could probably find yourself a decent 3.5in dual 2.5" out muffler and have a shop fab up the bends out the back. Magnaflow 12587 would be a good choice.
just use the stock 3.5" pipe to the muffler. for a truck that is just a family hauler non-mandrel bends on the tailpipes shouldnt make that much difference.
Most exhuast shops around me will fab up dual exhaust for trucks all day every day so it shouldnt be any differnt than what they would normally do on a silverado or any other pickup.