The problem with double cab pickups, is not so much the payload, which would probably be fine with 500l, it is the weight distribution, and specifically the rear axle weight limit. With a double cab pickup, all the weight is going to be over the rear axle, so you would prob have to go with a smaller tank than the payload would suggest, to keep it legal. With a standard cab pickup, the tank would be further forward, so the weight would be spread between the axles.