Big horsepower 318 builds

Look at it this way; the 340 is only 22 CI larger than the 318. I think it all boils down to heads. Get a 318 to breath, and I see no reason why 400 - 425 streetable HP could not be easily achieved. The jump to 500 HP might make it far less streetable, but with the right internals and good heads, it should be doable.