Are girdles generally added for big rpm or HP or torque

I have no personal smalkblock experiance, but i have a center weighted crank in my bigblock making about 900 hp. Mega block, had some cap walk, put in center weighted crank, no cap walk, 7450 rpm shifts. My guess is a cw crank might allow another 150 hp before block failure. Hmmm, now how well are those stock cylinders going to hold up?