Resonating cab problems

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My old rusted out cab with it's one inch "ventilation slots" in the
floors, resonates badly at about 900-950 rpms. Will this get any better
when I replace the cab with a solid one? I'm planning on using sound
deadener everywhere I can get it and removing the fuel tank from
inside.
I know the aqustic properties of these small cabs are not very good for
sound systems. But what can be done to muffle the resonating? More bracing?

Thanks.