Use a finer pen
You've got the general idea, but to confirm your accuracy would require tuft testing. About half the drag comes from the tires and wheel wells so it pays to consider those carefully.
I had trouble finding a picture of a B2000 without the bed and at the right angle. so this doesn't show as much plan taper as should be possible.
I'd give it a shallow curved cab-over about 4-6" thick at the center. There is a low pressure area just abaft the windshield header, from the angle of the windshield. Also the square edges of the cab top induce vortexes and you could soften those. So even though it would increase the frontal area marginally, it might show some benefit.
Here's a quick, easy way to find the length of the boat-tail necessary to accommodate the width of the cab at the back of the doors. Get some flexible strips, the longer the better—12-14'. Clamp them together at 45° ( 2x the optimal plan taper). Prop up the end and lash the fronts to the door handles. Then slide it back and forth to eyeball the curve. 'If it looks right, it is right.'
On my car, I get a little head start on the plan taper.