If you filled your truck with 0.55 tons of coal, I don't think you'd be too worried. That's the same thing I've done here, the speaker only weighs 10% more than the rated weight of the bracket. And the rating of the bracket has a safety margin that I'm within.

Jason

M80 HP v4, VP160 v4, QS8 v1 (3 in 6.1 layout), SVS PB12-Plus/2, Parasound Halo A21, Denon AVR-X4100W