[DRJDVM's '69]

I was planning on doing some stuff to reinforce the package tray with either a bar under it/inside the trunk and/or adding a big flat plate under the tray and then bolt through it. Then a 4 point harness for each passenger

The principle is the same..... Take the force placed on that one attachment and spread it out over a large surface area. No different than the way the stock belts work

Look at the stock belt hole in the floor..... It usually just has a big flat washer or plate to reinforce the floor