74 Dart Swing How to install backseat seat belts?

Heya Folks,
I am looking for advice from anyone with experience in installing shoulder/seat belts in the back of a Swinger. We have two kiddos and our youngest is a 3 year old who is in a booster/child seat...and will be for the next couple of years and are trying to figure out our best options. We also hope to keep our car as "stock" looking as possible...so aesthetics are important. ;) We are brand new owners of this car (as of yesterday) and although we have owned old MOPARS and vintage Chevy's before - it's been a REALLY long time...and we didn't have kiddos then either!!

THANKS!!!