Big Single exhaust?

My '95 Ram 2500 with the V10 came with a single 3" from the factory. First thing I did when I bought it (used) was put another muffler on it since the V10s sound like sixes. The muffler shop said, "Let's put a bigger 3" pipe on it to let it breath."
They didn't believe me when I told them it already had one on it - until they got it up on a lift and measured it. It's not an original California truck.