I don't think the flange at the top would help, and it might even make the problem worse. What you really have to be concerned about is the thickness of the deck between the sleeves. For example, if you put two 0.090" wall sleeves into a 427 block, and have to bore each hole to 4.375" in diameter to press in the sleeve, then you will have reduced the thickness of the area at the deck between the bores to 4.63" (FE bore spacing) - 4.375", or about a quarter inch (0.255"). Factory blocks do not have siamesed cylinders, and the deck is only about an inch thick, so you have this 1/4" thick, 1" tall piece of cast iron supporting the deck and adjacent cylinders. This is the problem. Cutting more off the deck to seat the flanges makes the area even weaker.
Now, one potential way to do this is to bore for a sleeve that is smaller in diameter so that you leave as much of that material between the bores as possible. I don't know what size sleeves are commonly available, but custom sleeves certainly are. If you could bore a stock block to a 4.25" or 4.28" OD, press in a sleeve, and then bore the ID to the required size, you would have a lot more material between the bores.
Usually, however, you sleeve a block because of some significant damage to the cylinder. So probably the whole original jugs were cut out, meaning that it was bored oversize much larger than stock, and left the thin spot between the bores. Any idea why the block was sleeved? Or better yet, what are the outside diameters of the sleeves that were installed? That would give you a better idea of how much material was removed between the bores.
There are more knowledgeable engine builders than me here who might be able to answer this question more effectively. For me, as originally stated I think I would steer clear of that one, unless I had a lot of detail on the sleeve installation and felt comfortable with that.
