[Blue Bowtie]

Don't over-analyze. The O² gets an exhaust gas oxygen content indication from whatever gasses pass it. If only three cylinders are exhausting past the O², the O² will report the EGO for those cylinders to the ECM, The ECM will adjust injector PW to maintain a target EGO and "satisfy" the O². That design system is intended to closely monitor and control fuel on those three cylinders, and presumes that the other three cylinders are operating under the same conditions, and will therefore react similarly to injector pulse width changes. It had been done like that for years, until SFI became prevalent.

Although a balance pipe (crossover) in a dual system can alleviate uneven pulse backpressure, I'm thinking that the even firing order of the Buick V-6 doesn't require it, unlike SBC engines which have adjacent cylinders firing sequntially.