Once helped rebuild a flatbed semi trailer used to haul aluminum extrusion where all of the original 4x4s had rotted out over time. Built two flatbeds for two new 70 series GMCs used to haul boats from a factory to the dealers, but were very heavy. All used wood 4x4s, lot of 4" "C" channel iron to support and to cap in around the outside edges. Were very heavy and heavy duty, but unlike a pickup, the frame rails were straight (continuous flat) from front to rear. Pickup truck frames dip down to go under the cab - we didn't have to deal with that problem.
Probably best thing to do is eyeball one somewhere and go from there. They're all over the road and a picture is worth a thousand words, and far better an explanation than I can type. Good luck.
I'm not familiar with DOT rules that say a pickup bed doesn't need to stop, but a flatbed does. But, my trucks have original beds, so have never been through that before. Could very well be.