Hey, J., SmokeMullet is sure to chime in here with some good advice.
Meanwhile, the time or 2 I've done mullet, I did a simple procedure. Basically, just coated the butterflied fish with a spicey rub, and threw them on the smoker...for not very long. Maybe an hour? Didn't take long.
I used hickory, but the Cedar Key folk love that red bay for mullet. Has a unique flavor. Can you get bay wood?