well considering my blowthru is designed like this just sideways with 4 12's abd i peak at 35 hz i tunned it to about 31hz with doing it the way i told him to do. My box EXACTLY what it was designed to do like every other box i've done does. And yes the tunning will be affected because of the way the subs will interact with the port and the air movemen butit really seems like your trying to make this more complicated then it has to be.
perssonally i dont know what your overall volumes after displacement is but i'd try making the mouth of the port a little smaller, that'll help with tunning