You need to differentiate between damping panel resonance and vibtration (which is what Dynamat and the like do) and actually blocking sound.
Dynamat-style resonance dampers aren't designed to be used as a sound barrier and it's only necessary to cover about a third of a vibrating/tinny-sounding panel for the damper to be effective.
To actually stop the road/engine/exhaust noise getting into the interior you need a contiguous barrier layer, which needs mass (e.g. 5 kg/m mass-loaded vinyl, lead sheet etc.). To gain maximum benefit this layer needs to be as complete as possible (taped joins, no gaps) and to be decoupled from the floor/bulkhead/whatever by a layer of something like closed-cell foam (which might also help absorb some high-frequency noise).
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