Just to let you know I have a MP .508 lift Purple cam in my '70 340 and it's got the rumpity, rump your asking about. That said, a .508 lift cam would be a little much on a stock 340 motor. You'd get some of the lope out of a Comp grind 280/480 cam and still be somewhat within your motors RPM range...just my $.02
Even w/ my cam and 11.1 compression the rumpity rump factor isnt excessive, but is very noticalble. It all depends on if you want rumpity, rump or RUMPITY, RUMP!