Stef, anything you do to your car can have a bad effect. It all depends on how it'* running right now. Any one of our cars could die tomorrow. Even with low miles. The 3800 has a reputation for toughness, so with this car, more than any other, if you use reasonable mods, and take safety precautions (like preventing heat buildup) you're generally ok. The smaller pulley would only add more stress when you're really getting on it hard. Normal everyday driving would be fine. Save it for that 'special' occasion. I firmly believe that if you take the right precautions to shed some extra heat, you can offset the extra heat the SC develops with a smaller pulley.
Heat is the #1 killer. And we know how to take care of some of that. It can be managed.