Four grade 8 stainless steel bolts with washers, bolted through the metal bumper or tailgate.
Red Loctite should keep the nut on. As an alternative, you could tack-weld the nut and bolt together.
Yes, even *you* will have a time taking the plates off. But it's nearly guaranteed that the plates will not follow our ancestors out of the sea. Having a plate stolen is a pain, as is having a tailgate stolen. Which reminds me, *that's* why tailgates often have locks on them today: not to protect the unprotectable contents of the bed, but to prevent the tailgate itself from being stolen.