72 Duster 3 point seat belts

Hi Everyone,

I need to put in 3 point seat belts all throughout my 72 duster on order of my wife to have the kids in the car as well. I have buckets in the front, and I found this kit from Seat Belt Planet that seems to just bolt in:

1971-1974 Plymouth Duster Driver & Passenger Conversion Kit

However, does anyone have any experience with adding in 3 point seat belts in the rear, bench seat? The package tray looks like it has a slot for the belt to run through for the shoulder, but I am not quite for sure how to mount it. The car is in the body work phase so if I need to have anything welded into that for this, now is a good time.

Any experience or thoughts would be appreciated.

Thanks!