looking to find specs on 340-X heads.

Fishy, your book is wrong. LOL, LOL. The fact ramains that in 1970 the 915 castings were introduced on all 1970 340's. I know I was around then. I pulled the heads off a buddies 1970 340 Dart swinger in the early '70's and they were 915 'J' casting heads. That was so long ago, I was still in the Air Force then but I remember it well. He had a bent crank. He never put it back together and he ended up giving me those heads. Don't want to extend this into a pissing contest but I know what I know. Try a different source and check that out. But as I said before, LOL,LOL...it doesn't matter which LA casting you have, use it . It is as good as any of them. We will just have to agree to disagree..and that is ok. Good luck,
Terry.