Rear seat shoulder strap conversion?


Anyone know of a shoulder strap conversion kit for older cars?
I'd like to take some family cruises in the GN but I know the wife will have an issue with the lap belts only in the rear for the kids. I don't want to do any serious modification, but if there is something available that is relatively easy to install I'd consider it.
Simpson used to sell a 4 point harness and they were only 1.5 inches wide. The lap belts could bolt to stock bolts then get a baby tether bolt/washer for the shoulder straps. I put a set in a 67 Nova for a guy years ago and his kids loved them.