Block wise, the 5.7 has a solid casting through the main webs were the 6.1 has a 'D' shape opening in the web. For windage??? I don't know. The 6.1 also has neat little oil squirter's to help cool the pistons where the 5.7's don't. Sonic checking both the blocks, the 5.7 can handle more of an over bore than the 6.1. The 6.1 comes with a forged crankshaft which will drop right in a 5.7 and also has 4 bolts that attatch the toner ring to the crank compared to the 5.7's cast crank with only 3 toner ring bolts.