FWIW I've read (I think in Hot Rod) a quote of Hank Cornelieson (sp?) about being able to get a reasonable ring package in 1.25 worth of CD (going with you meaning Compression Distance when you say CD). It had a recommnded minimum thickness for the top ring land, ring spacing and kept the oil ring wrist pin free, but I forget the exact ring size they were quoting, the specific spacings, and the wrist pin diamter (but I think it was SBC for the pin). Was it an accurate quote (IE did Mr. Hank really say that)? I have no way of knowing.
FWIW everything I have read says the trend is towards smaller journals. The NASCAR guys are supposedly (or were) running the 1.888" 'Honda' rod journal (Quad 4 if you're a GM fan). They spin those 9K + w/ crazy compression etc. I read the Pro Stock guys have been trying a 2" rod journal on their humongo big blocks (500 CID?) and they wind the daylights out of those too.
What does that have to do with a low RPM 4 main bearing straight 6 that needs all the rigidity in the crank it can get (w/ more journal overlap = more rigidity)? Hard to say. It would be worth whatever it cost to either have a crank X-rayed, or spend some qulity time with a sonic tester to see how much you had to play with.
Now, if you were thinking of Bonneville, that's a whole 'nother deal entirely.
Yes 2 FWIW's in one post is a bit much, but I'm not claiming personal expertise, just passing on what I've read, and pontificating a bit (it's why they're FWIW's).
