I think that I am probably on my own when determining intake length; I'm going to build a peripheral port engine and turn it about 7400 RPM which will be just short of 2600 prop RPM (hoping for about 230 HP but more is always better!).  Actually, Mazda has done a lot of research on PP so I should be able to get in the ballpark.  Again, the problem is that the ideal length changes based on temp and pressure (or was it pressure and humidity???).

Ken P.