400 B block

I would think the CI range would be in the 535" range. Limiting factor would be the overbore. The thicker the walls, the more cylinder pressure the block can carry. Any stock block is limited to .060" over, assuming the sonic test gives the nod.
The bottom end is as critical. You asked about main girdles, and rightly so. I would look at CRE. More expensive but they offer new aluminum caps that work with the girdle, if a bullet proof system is what you want. No worries about the bottom end...........
http://gearsandrears.com/girdle.html