custom headers

Im trying to determine my header collector location. my question is, is there any type of gains in torque or hp with a longer primary tube.im putting a 502 with p51 heads in a 64 merc, I have purchased assorted 2 1/8 od bends and pipe,p 51 flanges,and 3 1/2 collectors. I have seen some numbers regarding tube length on this site of 30 to 32 inch tubes, looking for any input. thnx....  

I built mine for my 572, they are 2" primary about 29" long with a 3 1/2" collector.
And it made more power than I was shooting for. So I can't say the headers are hurting anything.

Have you tried using pipe max?