By the time you pay parts and labor on sleeving 6 holes, you can buy a couple of core blocks. I do it at times on 350 V-8 race engines with lots of machine work on the block, but I've got a couple of thousand bucks' worth of labor in one of them. For a street engine, it's not worth the effort or the cost.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!