Rear shoulder belts?

I am looking to put some shoulder/3point seatbelts in the rear seats of my 68 Barracuda Convertible. Has anybody done this? If so, can you give me some advice or tips?

I haven't pulled the seats out yet, but thought I heard maybe in 68 there might already be some upper mounting holes for shoulder belts, anyone know if this is true?

My 7yr old son rides in the back, so looking to keep him secure. Up until now I've gotten away with a full car seat with a full harness, but he has outgrown it now.
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