How much power can a 440 block handle?

If you are concerning yourself with any longevity...Naturally aspirated, a good sonic tested block with proper prep will go 800hp or so for a while. Eventually, the main webbing will frature and that will be that. If you run a crank driven blower, again, with proper prep, I don't think 1000hp is out of the realm of possible for a while. But, why anyone would not spend on a much better block to begin with is beyond me. Because when they do fail, they take everything else with them...lol. The proper prep is caps, girdle, oiling system work, and possible hard block. By the time you're done, you have sropped 75% of the cost of a new better block and that's the cobbled together foundation for a big expensive engine. Spend the extra $$ and get longevity and rebuildability.