I used the seat back stops, because they seemed to be the strongest anchor points.
Do you guys have pics of where you mounted your retractors with or without L brackets? Can you please share?
The seat back stops are not "seat belt" anchor points.
Your location seems fine I would just add a set of L-brackets as doublers for the seat back stops and use the extra-large backup washers provided in the kit for the inner fenders.
This way every attachment point would have the 7/16” grade five or better minimum required hardware with back-up plates on the sheet metal.