Rem...Keith's reply was an excellent description of the difference...and G's comments...well said...
In the end there's no real performance difference in the blocks as Keith mentioned....
He also mentioned the roller cam bearings they use for some special applications...these are some images from a project 428 with this operation being completed.
The grooving tool
http://www.gessford.com/projects/ima...earing-A12.jpg
The machine work boring for the o/s roller bearings
http://www.gessford.com/projects/ima...earing-A02.jpg
and some additional reading materials here on 427's
http://www.gessford.com/images/fordsup-33.gif
http://www.gessford.com/quotes/image...erf1966-32.gif
http://www.gessford.com/images/427staged.htm