I have obtained set of Shoulder Belts that build sheet said My '67 GTX had,and drawings show that They are anchored thru the wheel well with a large
diameter washer that has a flange/shoulder around the outer diameter of this Washer, and a 1/2" bolt
thru the Washer into the interior of the Car. Anybody know of Someone who has these washers??
They are the same diameter as the washer/nut assembly spot welded to the underside of the Cars that rear seat belts bolt into..

This type of anchor plate or bracket is all I've ever seen used to attach seat belts.

Those are the ones which are in the driveshaft tunnel which mount the front seat belts.. Others which mount the rear belts are large round with that square nut.. I can use a pair of those, will remove the square nut..

Those are actually the inner rear seat belt mounting plates from a 75 Dart Sport. Similar plate in the rear wheel house for the outer belt retractor mounting. Also the same on the 76-80 F bodies.