For max hp with one stage diffusor:
-Header length should be 6-8 times header diameter.
-Header area should be 15-20% more than exhaust port area.
-Big end of divergent cone should have 6.25 times the area of its small end. Length is given by its taper (6-9*).
-Convergent cone taper should be 15-20*, length again given by taper.
If you follow this when building/altering your pipe - dwell section length is where you can tune in your tuned length.
(Extiming x 1700)/rpm