One sentence in your post jumped out at me: "I can turn the rocker adjustment screw about another half turn before it bottoms out..."
Maybe I am missing something, but the nut that holds the rocker down is really used to adjust your lifters. You are familiar with that procedure, right? Make sure the valves are fully closed (piston at TDC of compression stroke). Run the nut down finger-tight until the roller contacts the valve tip. Then turn it another 1/4 to 1/2 a turn and tighten the lock nut.
But that makes me notice another thing. It seems like your rockers should have lock nuts, maybe like the ones in this photo?
Go Mean Green!