Six pack on 318

4 bbls are rated at 2"
2 bbls rated at 4"

To get the same value for a 2 bbl rating to a 4 bbl rating divide the 2 bbl rating by 1.414 IIRC

A 1350 cfm six pac rates at about 955 cfm using the 4 bbl rating pressure

Headers make a big difference. Dougs and tti are the only ones that fit right I like Dougs better than tti based mostly on the fact they are less expensive but still a great part.