I have heard that if you use LS1/LS6 stock rockers with and LS2 block you need to put shims under the pivot plate for correct valvetrain geometry. Has anyone heard this? The link below is where I got this info:

http://www.cajundude.com/engineswap5.htm

My dyno numbers were a little lower than expected. This is the only thing I could think of that might have caused a problem.