You shouldnt have to forge the internals with the Mongoose kit unless you are running over 400 HP at the wheels. Tim has got cars as high as 420 I believe that still had the stock heads and internals, but that is pushing it a little I think. MPH has an outstanding reputaion for outstanding tunes that are very safe.
Bottom line..under 400 is pretty safe. Over 400...pushing your luck on a stock engine.