I have a bead roller and have done some bead rolling, but that is not how I would approach that project.
What are your plans to be able to bead roll the rear panel of that gate? Will you be separating the inner and outer? Or will you be graphing in a bead rolled panel offer it is embossed?
How would I do it?
I would have a panel plasma cut in order to create a male/female die that sandwiches the panel to be embossed. Then you run that through an English wheel or a slip roll to make the embossing.
There is another way as well, you can cut the letters out and then silver solder them to the tailgate. Then do the finish work.
If you want deeper letters you can cut the letters out and then create a hammer form to roll the edges and give them more depth, and then silver solder them to the tailgate.
Keep in mind that when embossing the metal has to come from somewhere so you will see some "shrinkage" and/or warping of the panel. This is a problem when trying to emboss into an original panel verses embossing a separate panel that you will later weld into the original panel.
As was mentioned above, if you can get an older panel that has the embossing you want, like from the 60s and then cut the piece out you want and weld it into your tailgate, that may be the cheapest way to approach it.
If you were local I would say come over and play with the bead roller, they are a bunch of fun.....of course once the coronavirus thing is over you could always come over on a road trip.
If you need more explanation or ideas on this just message me and then we can get on the phone. Also if you want to learn more about this stuff look on metalmeet.com
Bob