Is/was there ever a 340 Magnum

Yes, I'm pretty sure you are correct - except I believe that cam was only used in 1968 sticks. May have had a dual point distributor too. But the HP remained the same. They may have mentioned it in the ads but didn't make any comparisons suggesting that one was better/more potent/faster than the other.
Mellings and TRW used to have that cam available. But I can't find anyone who is producing it currently. My friend had one in his 68' Dart 340 4 speed car had one. It had a wicked idle for a stock camshaft.