The Ls1/6 block is very close to the Ls2. A Ls2 is a 4" bore and the Ls1/6 are 3.9" and the strokes are the same between the motors. The Ls6 and Ls2 both use 243 casting heads.
For more try Ls1tech:
http://www.ls1tech.com/forums/showthread.php?t=306577