If you had the money to build 500 rwhp why would you settle for a stock block. The thinking behind it makes absolutely no sense to me. If I were dumping thousands upon thousands of dollars into an engine to achieve those kinds of numbers there is no way I chance all of my money, time and life into something that is on the brink of disaster. Potentially losing everything because chances are the parts will not be salvagable.
Those magazine ones are probably guys who thought "hey lets get in a mag by building a 500 rwhp stock block Mustang". They do this with no thought of ever really driving it or keeping it that way. Its just to say look what we did and we got in a mag. ("Now lets tear her down and do it right").
As for limits there is a design theory of the block. There are also real world results. There are a lot of variables that go into how liong the block will last.
That said, my pal put down 368 rwhp and 400 rwtq. We then threw on a 125 shot. I would say that, that was awfully close to 500 rwhp and definately over 500 rwtq. He drove it like he stole it, spraying the crap out of it. Wanting to see what it could handle because he already had the funds for the new set-up (which is a dart block stroker). The mainshaft in the tranny blew not the engine. But like said above, that extra 125 wasn't there all the time