The long and short of it
From what i,'ve been reading collector length don't get much consideration, seems to be all about tube size, diameter of the primary's and the exhaust pipe.
1.625" Primary's and 2.5" pipe is good to like 5-550 horse.
I ran the optimizer in my Engine Analyzer +3.2 and it looks like long collectors help the mid a little bit at the cost of top end. long primary's on the other hand pay big dividends on top. I need a header for S-10 chassis thats got 41" primary's.