Shoulder bar help

[65 347]

Guys looking to install a shoulder belt bar in my "65 mustang. I saw this pic of a first gen camaro on another site and can't remember where I saw it ask him any questions.
What I'm wondering is what size bar and thickness should I use and how do I tie the bar into the inner quarter. Is this strong enough for street driving and autocross driving?
I would like to have a bolt in bar for the rare chance I take the family to a local cruise night.
It will be interesting to see the opinions about this.
Thanks for the help.

Mike