i don't think i would worry too much about it. just watch it as you fill it up. i know my 81 weighs about 3800# with me in it and the GVWR is about 6000# so i can haul about 2200# safely. now i don't see how an extra 600# would hurt anything if your not moving. by the time you have displaced some water you should probably be back withing the GVWR