Im sure there is some baseline for calculating this, but early manifold manufacturers usually just took a factory manifold and added more carb flanges or just grafted a 4 bbl flange onto them without any additional engineering or modifications to the runners or plenum and called it good. Until Clifford came along and began increasing the runner size and plenums in their manifolds that this trend changed. So for the Packard, I would likely follow the same path by using the factory runner length and volume since this is going the be a relatively low RPM engine, and just add additional carb flanges or have a one off intake cast for it.
