Well if you actually compare a 5.7 and 6.1 block side by side you'll see that the main webs on the 5.7 are built stronger. The webs on the 6.1 have a casted hole through them whereas the 5.7 is casted solid. The 5.7 blocks also sonic check much higher than a 6.1. Most 6.1 blocks can only handle a .020" over bore where most 5.7's can handle up to .075" and maintain the same wall thickness.
