Back when L&RP was printed on paper there was a large article about the first restoration of the 464 at the Huckleberry Railroad.
There was a great picture of the engine on a bridge showing they stand Rio Grande lettering. Caption said that the former owner had given permission to use the name?
I would say "go for it" A few minutes on the word processor of your computer and 37 centsto write a letter is worth getting it historically correct.
but whatever it ends up saying it is great to see the locomotive running again!
Ted Miles