3 point seat belt, '63 wagon

Hi!

I am about to install 3 point seat belts in my 1963 Dart wagon. For my two kids, five and seven years old.

Safety is top priority, of course. Or at least as safe as possible...

What kind of solutions is there around? Will the c pillar hold or rip if I go for this solution -> 3 Point Seat Belts Installation of Shoulder Anchor Point Where Door Post Goes to Roof

What have you girls and guys solved this?

Kind regards /Peter