Engle valve springs

I was considering using the factory style 340 valve springs until I saw this article:
1970 340 High Performance Mill Dyno - Mopar Muscle Magazine
With the concerns this thread started, I now have my doubts about that hotrod article regarding their valve spring conversion and the retainers they used. The article does not mention valve stem size. Are there really two valve stem sizes available for 340's (11/32 and 3/8)? If so, I wonder why?
Thank you.