Truthfully? Do the best you can without bankrupting yourselves for as long as reasonable, then take a deep breath. A some point this issue is going to pop up in more than just the truck and all you can do is hope for no crashing. My kids range from 5'3" to 5'10". We ran out of car seats quite awhile ago. Neither of my sister's vehicles has head restraints. My mom's car either. FILs truck is like yours. Numerous friends and people at church have older cars. None of these cars are daily rides for my crew, but with that many possible less than ideal variables in play, it does happen. While I could drive them everywhere and never let them ride with anyone else, I don't. For one thing, I'm outnumbered. There are too many kids going too many directions, plus my MIL that needs full time supervision but is rarely willing to leave the house. These less than perfect cars take my kids to valuable experiences and let them have relationships that are worth the risk. Keeping this in mind keeps me sane now that the only option for mitigating the risks is to cloister them, and ultimately, it's way less scary to send them out into the world with a trusted person that doesn't have head rests then it is to watch them drive away with their brand new driver's license.
You still have options, and I do encourage those options. I'm not saying throw in the towel, just offering some perspective for the future.