Set your engine, fan and pipe up in a test stand. Run the thing WFO while moving the the pipe back and forth. With your ear find peak rpm lengthen the pipe about .125" to .188" and you have your length. If you have a freind with a leath and a tig welder, you can make your pipe a fixed lenght at this point by leathing out a header form 6061 bar stock and heliarc welding it the pipe at the right lenght.

Dont try welding a cast header to a spun aluminum pipe. It dont work.

If you want to get more precise with the rpm paint a .25" wide line across the spinner dome on the fan a use a rpm gauge to read the rpm. Find the peak rpm and lengthen the pipe tell the rpm drops off 300 to 500 rpm.