Interesting article from Super Chevy, but they were wrong about GM producing a 307 with a small journal crank. Quite unusual for them.
Here's an excerpt from Hot Rod Magazine. It's the second paragraph.
https://www.hotrod.com/articles/ccrp...-engine-specs/
And here's a Wikipedia article on small blocks. Read the first and last paragraphs under the section entitled "3.875 in. bore family (1955–1973)".
https://en.wikipedia.org/wiki/Chevro...l-block_engine
Finally, go halfway down the page here:
http://chevellestuff.net/qd/crank/crank.htm
Bottom line is I think GM needed a new block for the 307 with more "cast-in" clearance for rods (due to .25" longer stroke). And why cast a brand new block with small journals, when they were moving to medium journal cranks on all other small blocks (302, 327, 350)?
I did find a list of crank castings at Mortec.com that shows only three castings used for the 307, and they were also used for 327s. So, they were the same raw castings but machined differently for balancing with different piston weights.
3911001....307,327...cast....medium journal...3.25" stroke
3911011....307,327...cast....medium journal...3.25" stroke
3941174....307,327...cast....medium journal...3.25" stroke
A couple pictures of my 307 crank rebalanced for 327 (+.030") pistons and SCAT rods: