340 cam in a 318

Yep. Which 340 cam are you using?
It's a 272/.455 cam, a little hotter than a factory 340 cam. So If I go back to a 2bbl, and the car is a little faster in 2bbl form than a stock 318 2bbl Duster, then that pretty much proves that you can add a cam, like a 340 cam, to a 318 2bbl and it will still add performance.