One of the other more prevalent problems is the rod bolts. Stock rods can be used to 7k or so, but the factory hardware stretch and grenade the engine on occasion.
So, per Milo's suggestion: replace the valve train (valves, springs, keepers, the whole enchilada) and the rod hardware.
Block split can be attributed to casting variation since Ford was relatively new to mass production thin-wall casting techniques. Very seldom do machinists find a Cleveland with everything exactly where the blueprint says it should be. Manufacturing variance made some blocks weak and some quite strong. Most are in the middle and do just fine north around 7k.
On the upside, there's a lot of old Nascar stuff and some really great parts coming out of Australia and New Zealand for the Cleveland, where it lived on years after it was given the axe here in the states.
The old saying "if you want to make 400 horsepower with a 351 Cleveland, just use good parts and start pulling plug wires until you get there," still applies as it always has.
here's a detailed build that you can copy for some very good and reliable power that won't be twitchy and need to be tuned every weekend or every time it gets humid.
http://www.network54.com/Forum/263038/thread/1315614851/last-1317319910/351+4V+C-4
The builder lists it as doing mid-high 12's in a Cougar sized car at over 4k feet of elevation. Not bad.