383 needs more power

The thing about dyno's is they are a tuning tool. You could drive down the street to a different dyno and gain 50hp, or loose 20.

Was any tuning done to achieve your output?

Tell us more about your combo. What is the static timing and total timing at what RPM? What ignition system are you using? Have you looked at the plugs to see if you lean or rich? Was the engine rebuilt?