Exactly, below .500 lift p/v clearance is not a concern.... Especially with stock low compression 318 pistons....
Yes, it's worth putting a 340 cam in a 318.... Just make sure to have dual exhaust with it for best results....
2 1/4" dual exhaust is fine... Get some nice street turbo mufflers so it's quiet at idle and rumbles when revved with minimal restriction.... Crossover or x-pipe is your option, but not absolutely necessary....