I have 2 RCI 5 point harnesses in my VR - Got them from Summit for $59/ea. I used the stock lower belt mounting points for the bottom belt and one side bolts to the stock location on the strut tower. The other side gets bolted to a strut brace I welded across the strut towers. It's not the ideal solution, better would be a roll cage with attach points on it. But for sub 150MPH I will hopefully still be alive (if I don't roll).
You could try a 4 point harness, that way you don't have to worry about the crouch belt(and the problems related to the stock seats)but still get more protection and a more positive locating belt than the stock belt system.
I put the Simpson harnesses in my '82. I had to make some mounts for it all but it was worth it. I used some peices of 1/8 inch plate with some chain links welded on to bolt the seat belts to the stock hard points. Then for the shoulder belts, I used the stock hard points again and stretched a peice of small diameter tubing between them, used 1/8 plate again to give me something to weld to then bolt on. So the whole thing mounts to all the stock hard points with the stock bolts. The sub belt I used a peice of angle iron and another small peice of 1/8 plate to back it up, drilled a hole in the floor board and bolted it in. The whole thing locks you in tight when you want or you can loosen them for driving around town. I have them on both sides.