Most of the 1 & 7/8 headers will come withy a 3.50 collector. Every time I have done a back to back test, the larger headers will loose about 15 to 17 pounds of torque, and the same in HP in the higher RPM range. Even up at around 700 HP, the smaller header is better up till about 7000 RPM. The higher the engine can rev up to, the better the chance of the bigger header showing an improvement. Now if you can rev up to 8000, the larger header will look much better yet.

Many people OVER THINK this whole header size thing. On the dyno, it is pretty much black and white when you compare differant sizes. Many times, it is much harder to see gains or losses in the car, because too many other things come into play. My experiance in the car has been that the collector length is the most important thing you can change. I have seen where the priomary length is way too long, and this can loose you as much as 150 HP=== THAT IS CORRECT=With SB Chevys, anything close to 28 to 30 inch primarys will worl pretty good. Case in point. About 20 years ago, an engine customer brought his car headers to run with the engine I had just built for his car. They were Hooker super comp, 1.75 by 3.00 ( very normal stuff )I noticed that the primary pipes looked very long, so I measured them. They were all about 37 to 38 inches long ( TOO LONG )all the Hooker catalogs listed them at 30 inches long ( WRONG ) This little miss information would be responsible fo a 50 HORSEPOWER LOSS . I called Hooker, and they said I was the first one to ever notice that=== HOW BOUT THAT. I cut the guys primary pipes back to the last bend, which made them about 5 to 6 inches shorter, and this picked up about 33 HP over the longer 37 to 38 length. Lesson learned here, double check everything, and dont just go by what the catalog ( or what someone tells you )

Collector length has more to do with power below the point of max torque, say from 2500 to 5000 RPM, If you have the wrong converter stall speed, the collector can be a big help

According to Jim McFarland, regarding header primary pipe size.................Exhaust system Peak Torque RPM = (Primary Pipe ID Area x 88,200) / Single Cylinder Volume in cu. in.
And the induction side of the engine could very well have a different Peak Torque RPM as a function of runner & port area. Average the two, and the engine dyno may show some pleasant results.