The standard-bore block would be a good choice if you can get it without paying a king's ransom. I have run blocks with those kinds of cracks on the round track at HP numbers far above stock by brazing and using helicoils to reinforce the threads, but we didn't expect them to live very long- - - -half a season, usually, and maybe 500+ racing miles before they hand-grenaded.
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!