One advantage to starting with a 3/4- or 1-ton frame is that the frame is narrower under the bed than on a 1/2-ton, making room for much wider tires.
However, what I was referring to was fit of the tires outside the bed sides. With the stock wheels and tires on your El Camino frame the distance between the inside surfaces of the tires may be too narrow to allow the bed to drop between them.
Not a problem if you plan to raise the bed floor and tub the bed.
