Here is what I did in my Coronet for rear shoulder belts. I used the 3 point conversion belts from SeatBeltsPlus at the link below. But I had them customize them so they would work for the rear. I ordered the customized belts for the rear from them in November 2024. At that time, the customized belts were $208 for the pair. At the same time last year, the belts at the link below were either $289 or $299.
1968-70 Mopar B Body Seat Belts - 3 Point Conversion - Bucket Seats
The part that the belt slides through for where it would typically mount at the roof for the front seat, I had them just use one of the small oval pieces that the belt slides through, like in the picture below.
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I attached this oval piece exactly over the slot in the rear deck. This way when the belt is pulled out, it rides against the smooth oval piece instead of the slot in the rear deck. It does hold the rear package tray up slightly more than the insulation, but nothing that is noticeable when the seat is installed.
The retractor which SeatBeltsPlus use locks via inertia. I mounted the retractors to the bottom of the rear deck. After the belts were mounted, I tested them a few times by giving a quick tug on the belt and only about 1-2" would come out before the lock engages. Also before I mounted them, I added 1/8" thick steel to the bottom of the rear deck for some added strength. The retractors are located so the part where the belt exits is centered in the rear deck slot. I did this so that it couldn't rub on the rear deck and tear up the belt.
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Where I cut the rear package tray for the belts to come through, I used plastic trim pieces from a Subaru WRX. They are black, pretty small and clip nicely into the rear deck. Here is a link to them.
https://parts.subaruonlineparts.com...pMJ7926NDzrXdW-W19AB7ZQ5cxBJNWqjEO-ANSuKpT9dM
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