Honestly to mount the harness properly you need some kind of roll bar, maybe even a 4pt roll bar for instance. Never use the built in anchors they are only rated for child seats. As for the lap belts, mount those to the floor board using the proper eyelets anchored by steel mount plates on the under side of floor board.
We used SafeCraft 5 pt harnesses on our track cars and in our race cars. I'll have to look for some pics of the lap belts mount locations.
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