**600hp sbm block limit? Or is it bs!!**

You'd need at least 100's preferably 1000's of examples of engines producing X amount of hp over number of years preferably in racing or another high stress usage with an acceptable failure rate, anything short of that your guessing.
It would still be speculation, there are no guarantee's with anything. Jimmy's block was good for 700hp lasting 100's of runs, but Jonnies R block exploded after only several runs