3 point belt in rear seat of 67 B Body Ragtop

So to make the rear safer for my son and the silly lap belts, I am toying with the idea of adding a 3 poing harness in the rear. It is from Wesco and the bottom two attachments will be in the stock holes. The top will be in the top rear of the trunk well right behind the rear seat. I know there are dangers in mounting below shoulder height. This should be close to his shoulder height and will come up over rear seat.. In a major wreck this should be safer then just a lap belt. I drive very timidly in this car with family inside as a flip in this car and everyone is dead.. I wish I could fab a roll bar right up thru the top well that didn't interfere with the top..

Any thoughts??

I don't really have anything to offer, just that this has been a concern for me for several years. I kept my daughter in her full harness seat for as long as possible. Now, I just make sure we stay off the highway, and out of busy traffic. Sometimes I wish I had a hardtop instead of a convertible.

Again, not any help, but I'm planning on a custom cage for the rear of my K5 with shoulder belts for the kids. Unless I trade for the FJ40, then the K5 will either be my toy, or get sold, and the FJ already has a roll bar that I can hopefully add shoulder belts to.

I welded in a four point cage without cutting up the stock interior. My new seats are in, but I haven't put the 3-point belts in yet. They bolt to the rear seat belt bot hole and can be quickly unhooked for rear passengers to use the stock belts.

Seats bolted tight in with a pre-made bracket.

Here's the belts. I opted for the self retractor, so I could lean forward and turn on the A/C and such. I hate having to unbuckle to reach something forward in the car.

They disconnect near the top and bottom on the back strap that bolt to the rear seat belt holes.