I don't know the exact weight just that the 6.1 aluminum blocks are advertised as 100 lbs. lighter than the production cast iron 6.1 blocks.

Not really apples to apples comparison, but when I swapped out my cast iron aluminum top end 340 with the current all-aluminum 426 Gen3 it was about 80lbs. less on the front end.

Yes, the engine was very expensive but I am so happy that I got it. No issues with it whatsoever, has ran great for thousands of miles with no major issues, very responsive on a road course and best of all no leaks!