Light weight bed to max out 26000 lb truck.

We want to change the 24' bed on our 26000 lb truck. We want the strongest light wt bed so that we can carry the heaviest payload and stay under 25999. How much heavier is a wood bed than steel? (I know aluminum is the lightest) Are there any disadvantages to having an alumium bed? Is red oak heavier than white Oak? How important is using kiln dried oak? Is a steel bed the best? I think the truck weighs about 12K without a bed. I understand that a 24 ft bed can weigh anywhere between 4K and 8K. We want a strong bed, but we also want the maxium weight bed. Can anyone direct me to some instructions that include Gov't or DOT regulations that would aid us in either building or a buying a bed. Any other thoughts? Thanks, Granny