The IROC-Z's 5.7L (350 cubic inch) TPI V8 engine was designed to run on regular unleaded gasoline. Using higher-octane fuel like Plus gas won't hurt it, but it won't provide any performance benefits unless your engine has modifications (such as performance chips or higher compression) that require the higher octane to prevent detonation (knocking). In fact, it might slightly reduce fuel economy since you're paying more for a fuel your engine doesn't need.
Check your owner's manual. It will definitively state the recommended octane rating. If it says regular unleaded, stick with that. If it's recommending something higher, then using Plus gas would be appropriate.
