Turning a 340 crank, How much is to much?

Unless you're really going to thrash on it, I don't see any problems with going to .030" under, especially with a forged crank. Mopars definitely don't have the smallest journal sizes out there, lots of engines around that start off with smaller journals/bearings than a 340.

One thing you might consider is having the crank nitrided though, the more you turn the crank the deeper into the hardened journal you'll get. It still probably won't be an issue, but for piece of mind you could have the journals hardened again.