I bet she hasn't been in your bronco either, has she? My wife loves riding with me. Specifically because it tends to lead to ice cream.
This is going to come out harsh, but it's reality: She needs to live a little. Call it a stretch, but she's attempting to emasculate the boy.
Today's safety standards in no way guarantee safety. In today's world, drunks walk away from their 70's trucks after a head on collision with a brand new truck. It's illogical. So long as your brakes are fully functional, your biggest concern is other drivers around you. My biggest problem is the way people drive when they pull up beside me and slow down to look. With their eyes off the road they ease over into my lane.
Put the boy in the truck and get some ice cream.
Also, you need to offroad your bronco, or you may as well just trade it for a 60s mustang, which may actually be cooler--and she'd probably let the boy ride in that.
BTW, I pulled the middle seats out of a Montanna minivan and built some brackets into the bed floor of my bronco. These seats have built in 5pt harnesses. And sure enough when I got them installed, my son was too big for them, so I had to install shoulder belts in the back seat.