I would suspect somewhere in the bottom, such as the meat around the main saddles, or the main saddles themselves. No girdle in the world is going to stop the inevitable. Hold it off for a while......maybe, but if you build an 800-850HP engine with a stock block, you better have another one waiting in the wings. How much boost do you think it will take? Headin towards if not over 20PSI is my guess. A stroked 360 (408 or 416) could make the power more easily with less boost, but it's still WAY more than a stock block can handle. Build it. It'll be a bad **** for a while.
