Actually, if I understand right, the only Series III engine is the supercharged engine, or L32. The L36 is naturally aspirated, and still a "Series II" design, if I recall properly. I may be wrong on that, but I do know that the power output for the 2005 N/A 3800's is the same. What the OP may want to do is research a L67 install. That's gonna be a bolt in job, if I remember right. Those SSEi Bonneville's had them from the factory. The website to check on that stuff is
www.l67swap.com if he or anyone else is interested. Quick way for him to pick up about 65HP.