I don't know much about the 85, but if you can get your hands on an 88 - 98 stepside bed that would really look good.
On the 1973-87 boxes ( full, med, step ) the gas tank set up is, to go half under the cab on the out side of the frame & half under the box.The filler neck on 81-87 should be on driver side unless it set up for duals tanks. I not sure if that helps or not. Was not sure if your asking about the 68 cab or the 85 step box. So since I know the 1973-87 better answered on it.
