player1up, once upon-a-time Brock Yates wrote an article asking if people would drive safer if it was a sword which popped out of the steering wheel. It really posed the question of offsetting mentallity. The insurance industry did a study published in Auto Body News (or one of the other trade mags) which actually backed up Brock Yates. People were braking later at stop signs, etc, thinking that ABS would bring down stopping distance. Not what it's actually for: keeping a car under control in slippery stopping conditions, they actually believe it will stop a car sooner. Younger people have started tailgating more. The airbag will keep them from getting hurt, right? (I actually increase distances when I'm driving the wife's car. I've had an airbag in my face. I didn't like it.) So when you speak of the other guy, you're entirely right. I'm speaking for the safety of our vehicles. They are, without a doubt, entirely safe.
I will agree with you on the peace of mind issue for your wife, though. If she has it in her head that your car is unsafe she will never be satisfied with the idea that our older cars are entirely safe, even if it's proven to her.
So, I'd consider rear shoulder straps out a car if you can bolt them in to a reinforced structure on your car. Just bolting them in on the package tray isn't going to be any better than not going without them, as a matter of fact, it could be worse. If you're going that route, grab the shoulder straps out of something like an older Tauraus and get the reinforced area on the sheetmetal to go with them.
Case in point. The rear shoulder straps and retractors out a Taurus are going in the old man's '54 Ford. The area in the OE Taurus package tray is going to be fitted and welded into the package tray of the '54. This will keep the retractors where they need to be in a crash. It also serves another purpose: the package tray cover itself is nothing than just an upholstered piece of cardboard, trimmed to fit right into the '54. Pop's going this route because he has the interior out of a Taurus and the package tray cover has the contour of the backseat.
If you can find something along these lines, with a package tray cover roughly the same size, you should be able to use the cover itself, too, to cover up the retractors. I don't know what's out there, as I've only had a wrecked Taurus on hand that fit the bill for what Pop wanted. A trip to your local U-Pull-It boneyard with a measuring tape and an idea should be able to help you find what you want.