[jhv48]

My Roush 427SR (rated at 515 hp from Roush) made 440/460 rear wheel hp/tq after I changed to a double pumper carb and 3" side pipes.

And, yes, the pipes can strangle that engine. Insert a small mirror into your side pipe and shine a light inside. Check out how big the internal pipe actually is. My spf side pipe measured 1 3/4" inside diameter and was choking my engine. Make sure your timing is spot on as well. Mine ran better at 36 degrees total advance.