340 improvements with dyno numbers

You're talking 1972 or later than. Stock 340 up to 71 was 10.5:1 compression. The engine mentioned in the article is a 1970. With 10.5:1 pop up pistons.

And yes, X heads can make more power than aluminum dart heads. It's never as simple as "bolt on new heads make power!" it's about matching your components. For example, running 9.25:1 compression on aluminum heads doesn't make sense. With the higher heat tolerance of aluminum heads, you have to up the compression. Or you lose power. Running less than 10:1 compression on aluminum heads is a waste. In my opinion running less than 10.5:1 on aluminum heads is a waste. Why run iron head compression on an aluminum head? You'll make less power.