Hello, and big thank you to SBBARRACUDA, I followed his ideas and successfully installed front shoulder belt attachment points on my early 1968 Barracuda. My early '68 had the shoulder harness bracket with a hole in it but did not have the nut welded on the back.
I did as what was recommended . The only real addition was that I made up a gauge to do trials with the puck prior to doing the actual install. That allowed me to perfect a method to hold the puck in position while the puck set screw was tightened. The gauge also served as a template to mark the position of the hole to be drilled in the car body for the set screw to match to the puck and to slightly enlarge the OE bracket hole. In doing some install trials with the gauge and the puck I quickly found that all I needed to do was to use a thin string, knotted through the main shoulder belt bolt hole at 12:00, insert the puck through the slightly enlarged car body bracket hole, then pull back on the string. the puck just about centered, I used a marker that had about the same diameter of as the shoulder mount bolt to align the puck set screw hole with the drilled hole. Ran the set screw in and tightened it slightly, used a razor blade to cut the string, finished tightened the set screw and I was done.