It looks like it was a running change, not a 340 only timing cover. 68 parts manual lists the same cover for 273, 318, and 340. I do have a cast in passenger side timing cover cast in 68.
my 69 fastback is an early build 318 car and it has the cast in cover, the 68 dart with a 273 is a later build car and it has the cast in cover as well, the 68 340 i had did not have a cast in cover and was a mid year(ish) build (and 100% virgin motor), and the very early 69 340 i had obviously had the cast in cover. the current 318 i have in is a mid year 68 production, and while it has the cast in cover i can't confirm that it is original to the motor-- and i'm too lazy to go check and see what cast date it is.