Ah, headers................books have been written about this subject. My Cad (4600 Lbs 454 Chubby, 440HP) uses 1 3/4" promary pipes with a 3" collector that's fourteen and a half inches long. It then flows through 2 1/2" mandrel bent pipes out the rear through Dynomax super turbos. When building (or buying) headers that will be used for a mufflered system it is far less costly (HP wise) to err on the small side. My system makes 2 HP less through tje mufflers than it does with open headers.

Having said that I reccomenf 1 5/8" pipes into a 3" collector thats at least 16 inches long, and 2 1/2" pipe from there back.