Is/was there ever a 340 Magnum

I could be wrong but. I'm pretty sure that the 1968 and the 1969 340 4 speed cars had a different camshaft than the 340 auto cars. It had a little more duration. If I remember correctly.
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.