Just use 3/8 or 1/4 steel. You'll need to take some measurments with your truck level. You need the outer diamter of your tire and the diameter of the axle.
What we did is put the truck up on jack stand and and made the frame level by adjusting them. Measure from the lowest point on the frame to the ground and record that measurement. Now add that to the Radius of your tire and then add that to the Radius of your axle... the bottom of the notch will have to be from the bottom of the ground to that height. It's kind of a crude way to do it but it works, might want to leave a inch of leway.