I ended up deciding to make up a 460 keystone plate. I removed the 68 keystone from the other smoke box front I got, painted it, and cobbled together numbers 4, 6, and 0 (used the 9 from the dry transfer leftovers for the 6, and the 4 from the "1347" included with the transfers). The size and shape of the Keystone isn't exact right I think, it's a tad short, and the numbers are a different font and size than on the prototype but I am relieved and satisfied.
I noticed that since the new Lionel legacy PRR 460 E6s has arrived, the smokebox front is now technically available on Lionel parts, but it says to call Lionel to order. I imagine they need to preserve those they have for actual repairs for people who bought that engine.
