Thanks to your responses, I think I found the bed!!!
'79 Datsun 620 "Step Side" kit!
California-stepside.jpg

Those are (I think) precisely how the bed and butt end of that truck laid out, including the placement of the telescoped tail lights!
Originally when the truck came to our family, the rear wheels were fat boys with turbine mags on all four wheels... *just* like this picture!
On thing they did with my D50 was to lower the stance in the front (or raise the butt) a little, maybe a 1/4 to a 1/2 inch... just enough to make the body line slant forward.