It's hard to know what exercises to recommend if we don't know what resources you have available to you.
Do you have any of the KTVA Stages? There are tons of really challenging workouts on the CD's and videos, and for ROCK, KTVA is about the only program that really focuses on belting and wailing. If you have Stage Three, just about any of the workout exercises are going to get your Rock Chops chopping! But these are not for the faint of heart! Nor are they for the uninitiated who have not yet fully paid their dues by working out dilligently at Stage One and Stage Two!
We cannot walk into the gymnasium on our first day and immediately bench press 350 pound barbells. We have to take the initial stregthening stages first, and then, when we have prepared our bodies for what is to come, we carefully and skillfully perform the task we have trained ourselves to do. I recommend the exercise on track 11 of the Stage Three CD workout.