Today, dollar for dollar, is the 318 faster than the 340 ???

Not even close. I go too fast with a 273, and I drive on the street all the time. I have had them all, 273 340 and 360, in a 64 and 66 Barracuda, 4 speed cars, with Formula S suspensions. They don't go to the track on a trailer and run a quarter mile at a time... LOL