I never like to use larger than 4.630 for the flange O.D. They just kiss each other at the tangent of the sleeves if side-by-side. Not enough meat in the deck like the aluminum blocks have. We sleeve many "used up" 427s back to 390 and 428 bores, but I don't like to sleeve adjacent holes back to a 427 bore. The cylinder behind the sleeve is the support that holds the flanged sleeve, and recreates the vertical deck integrity. The casting thins up as you get further from the original bore, so I don't like to go too big with the sleeve O.D. as it will eventually cut the deck loose. I have done some strange things to create support, anchored back to the floor of the water jacket, but it is time consuming and not practical unless it is a "special" block and/or there are no new alternatives available.