Well, it used to be real easy to add a rack back when they made cars and trucks with a "drip cap". That was a gutter that ran around the top of the vehecil. You could just clip onto it and go. I still have a set of bars that I used to clamp onto a 79/84 olds and then later my pickups to haul canoes....also had a luggage rack(box) that did the same for camping gear....oh, the good old days before the government got their fingers into everything.
You could redneck it.... just drill the holes in the top, poke hole thru the headliner, stick the bolts with a "body washer" on it up from the inside, smear some silly-cone caulking around it, put on another "body washer", add the rack and then put the nut on the outside.....or just Red Green it on with duct tape.