Couple of issues here. If you want to run the shoulder harness, you'll definitely need a "roll bar" with a horizonal brace. This is for two reasons. First, in running the belts rearward, tech inspector is correct; you need to direct the force rearward instead of downward. You can run a second horizonal brace below the first to attach the belts to. Second, when using these style belts, you need roof protection. These belts will keep you planted. If you roll the car and the roof crushes, your head would be the next form of support.

I would also try to upgrade the seats if you go this route as well. Newer seats are considerably stronger than OEM seats of the '60's and '70's.

I saw a guy autocrossing his Neon and he rolled the car. He had no cage, and with his belts, he almost broke his neck. His helmet was cracked.

Now in running a cage or roll bar, the rear seat will be useless. You might want to check some mid-'70 X-Body cars to see if you can retrofit an OEM three-point seat belt if you don't want to run the cage.