any 6 pak gurus out there?

What, You think they rated 2bbls differently "just for six paks"?? Nooo, they didn't. They are rated at 3"Hg each just as any 2bbl. The total rating of 1350 CFM is still at 3"Hg
depression, the conversion factor is 1.4142135 ( square root of {old depression/new depression}). The Sq. root of 3"/1.5"(std. 4bbl) <2>. 1350/1.4142135=954.5942 CFM.
You have 950 CFM available to maintain a 1.5"Hg depression in the intake manifold.