The 4.3 and 5.0 are the friendliest with 10:1 compression ratios. Those ratios can be considered low in today's age of advanced timing control and computer regulated methanol injection. Kleemann supercharged 5.0 boats, i mean sl500's, have trapped in the mid 12's at 7.5psi.
Apparently this is a CLk55 with every Kleemann engine mod and some internal work:
http://www.youtube.com/watch?v=DinXxTzaLgg