I can remove the tailgate and place the boards on the bumper as a ramp. The boards rest in place without needing to be fastened.
When I originally cut the boards down from 8ft to 6.5ft, there were 1.5ft pieces left over. I screwed those leftover pieces to the boards near the ends using 4-inch wood screws to provide additional support where the boards meet the bumper.



It's low-tech, but it works. The key was to remove the tailgate, which is easy to do with this truck.
Previously, when the tailgate was installed, I had been resting the boards on the edge of the open tailgate. But the boards were proud of the tailgate, so when the load was winched to the top of the boards, the boards would tip/move and fall off the tailgate. I solved that issue by installing eye bolts and using ratchet straps to secure the boards to the truck bumper via the eye bolts. But I realized afterwards that I could just remove the tailgate, and the boards would fit better without needing to be secured. So, the eye bolts are no longer necessary.
An additional benefit of the no-tailgate option is that the boards sit lower so the slope is more gradual.
For future consideration: Use 3/4-inch plywood as a platform or as a sled on top of the boards for objects that wouldn't slide up the rails easily.
If anyone's wondering, the winch isn't permanently wired to the truck. I use jumper cables to connect the winch to the battery as needed: Energizer 30 ft 1 Gauge Quick-Connect Booster Cables. The jumper cable clips fit well onto the winch controller's ring terminals.