Nice. The 3.5 "Shortstar" is a PAIN. Derived from the high-output Caddy Northstar V8, it's got a liquid-cooled alternator. That's just the start of how nasty they are.
Awesome engine/tech, but a bear to work on. Check out some stuff/tips/posts by 98LeCouch - he's got a 3800 Series II almost identical to you and has pulled into the 30's on the highway with it.