Installing a harness

My 68 Dart has only lap belts as well, Jan 1, 1968 was the date all vehicles in front outside positions were required to have lap and shoulder harness. I was looking at doing the same thing, but without a roll bar to attach the harness to, I am not too sure how safe it would be? But anything would be better than 48 year old lap belts. I have seen a 4 point harness go back to the package shelf on some road race type street cars. They used a 2 inch piece of angle iron under the tray and 2 eye bolts for a set of harnesses to hook to. They only used them at the track and unhooked the belts when done so they could use the back seat. It would be ok for a head on type crash but not sure it would hold you in the seat on a side type crash?