Exactly why I'm going to top swap mine. I have a hybrid Series II/III. In 04 when GM introduced the Series III, it became the basis for all 3800s in the subsequent model years. So, for Impalas in 04 and 05 which still used the Series IIs, they got a Series III block, bottom end, and heads. Only differences are the UIM/SC, fuel rails, TB, and electronics. Otherwise I already have a Series III in Series II clothing.