Sandbags in pickup bed, placement?

I have a 97 Ram 1500. I find it best to put 7-8 70lbs sandbags in it and over the axles. This way it puts the weight directly over the rear tires. This way the weight is pushing the leafs straight down instead of the rear half of the leafs being pushed down only. The idea is to plant the rear tires without lifting the fronts.
The reason I went with that many sandbags is I found that 200-300 lbs really didn't help much. I would still spin the tires from the spot lights.

My Ram is a 4x4 but with this much weight for normal street driving I don't need to put it into 4 wheel drive until it is real deep. I put a thick piece of plastic down(8 mil) covering the bed. I place 7-8 sandbags on it over the axles. I then cover them with the rest of the plastic. I tuck the ends under the bags so it doesn't blow around and come uncovered. I have used the same bags for 4 years.