These same engine-builders have found power by running less valvetrain preload. The general consensus is that power exists in valvetrains with a preload of about 31/44 turn of the holddown bolts. On a stock head, this would mean you'd run about a 0.040-inch shorter pushrod to achieve this-but always measure your valvetrain for this measurement. Most of the aftermarket valvetrain companies offer Gen III pushrods in multiple-length increments to allow geometry tuning. You could buy adjustable rockers, but they will just add valvetrain weight (not good for high-rpm applications) and cost that can be rectified with the proper pushrod length.
The LS6 heads were first released in '01 on the Z06 Corvette and are available today in assembled form from GM Performance Parts (PN 12564824, casting number 12564243) loaded with hollow stem valves (these were factory pieces from '02 to today) and LS6 valvesprings.
To learn more about the Gen III V-8, check out www.cartechbooks.com to order How to Build Performance Chevy LS1/LS6 V-8s.
Power SecretHere's a secret: If you want the best heads for performance, make sure your heads have these small triangle marks below every spark plug hole. The marks are left by "chills" used during the casting process in GM's semi-permanent mold (SPM) tooling. Of the three casting facilities that make Gen III cylinder heads, two use SPM tooling, the other heads are sand-cast. The SPM tools produce heads with very smooth, consistent ports and chambers which consistently dyno at least 4 to 7 hp better than the sand-cast heads.