Looks like the 340-6bbl, at least the auto's, have had some power taken off them at some point.
I was looking through an old "nitro joes" stat books(2010) and saw where they used to be factored at 328hp, now it's 319hp.

The 340-6bbl stick is now rated/factored at 2hp less than the 71 340-TQ...... 312 vs 314.