Where is the best point to insure the block is fully drained? or Should I just add antifreeze to the block?

Run antifreeze through the block then you don't have to worry about it and you get corrosion protection for the layup too.

There are hex key plugs for the water jackets on both sides of the block. A larger one on the left side in front of the motor mount and a smaller one on the right side behind the motor mount.

Thanks Gents! This LS3 is brand new and has not been run yet. Still working out all the details.