I'm not sure if the LT1 is aluminum block or not...but I know the LS1 is. And b/c the LS1 is aluminum (and since it doesn't have DOHC valvetrain mess), it weighs about the same as the VG despite having nearly twice the displacement.
I think an LS1 conversion would be awesome in a Z...simple reliable horsepower and torque with great response and decent revability. It could make for a very nice roadcourse monster.