I would feel safer going 60 over than running a sleeved 500+hp block with the 2"+ crack in my block (see pic). The guys at Dandy Engines are famous for their aluminum 1000+HP gm and fords but have no experience with iron mopar 340's here in Australia so I'm falling back on the experience of the guys back home who've been there, done that hoping for some solid advice.
The 40 over isn't my worry, spending big $ building a race motor with a crack in it is.