I called Dynamat's tech line yesterday morning. I asked their employee what area gave you the "best bang for the buck" and he told me to apply the Dynamat Xtreme to the back of the cab wall first and then the doors. He suggested covering the floor of the cab after covering the cab wall and doors.I have used but not in the gmt 400.
from what my ears tell me the worst place for noise intrusion is the back cab wall itself, the rest seems pretty decent (assuming you have carpet and headliner)
eventually I will ”dynamat” that area with a butyl rubber product, not an asphalt/tar one (peel n stick is tar) from a home improvement store. The idea is additional mass lowers resonant frequencies, and the elastomeric properties of the rubber binder absorbs it, while the semi rigid foil backing provides a decoupling layer.