.60 over is a big deal on a 400 when you're pushing all that power, those siamesed cylinders are going to create some hot spots, especially with those thin walls....... but if you're going to do it, i would spare no cost on the bottom end strength, go fully forged, also the top end is going to need to hold some pressure, o-ring the heads and install the best darn gaskets you can get your hands on...... if you build it right you can handle some boost on that high of a CR but just dont try pushing too much boost