At what horsepower do big blocks need a main girdle?
Answer: They wont hurt at any level.
The question should be, how much power can a bb make without a girdle?
Mine made around 1100hp with modified factory caps and a modified factory crank. No girdle. That engine was a 400 based 470 with turbos. There was a challenger with a procharged 383 making about 1200 with a factory block running around back then also. I can't speak for his internals, cause I simply don't know.
If I weren't so cheap, I would have put a girdle on mine. Here's what it takes to hold up to that power.... (yeah it's ghetto)