I can't find the large article I did some years back about the undersized headers most Superforance cars from about #2700 back have. Most don't even realize that the headers they have are for a 302 small port stuck on their 402's and 427 small blocks causing a huge bottleneck. Additionally, all Superformance come with two inch mufflers(inside dia) which also chokes them to death. Christ...my Harley has 1.75 in. exhausts for each cyl. For 500 HP you need the newer Superformance 402 headers which actually fit the AFR large port heads and replace the exhaust with 3 inch ID.
Anywhoooo...If you will send me your email I will send you the photos of the two different size headers where the ports don't match and photos of the sectioned Superformance mufflers. From what I understand, a 400 HP motor needs 2.5 inch exhaust......500 HP needs a 3 inch exhaust....and 600 needs 3.5 inch exhaust. email me at
wanab5150@comcast.net for the photos.