I have an isolated house battery system where all the grounds run through a shunt, then back to the truck battery, that way I get a better sense of the state of charge of the house battery and avoid potential gremlins from chassis grounds coming loose or breaking.
That means running duplex wire, which obviously costs more, but the labor is the same and the results are more reliable.
Otherwise, find your biggest battery cable and use a similar size to bridge your flatbed and the frame, you should be good. You might do well to tap the holes for both ends, use as fine a tap as you have patience for--
more threads= more surface area= better ground.