Why do I keep breaking rocker arms?

I'll put it to you like this. When my machinist finished my prototype slant 6 closed chamber head, all the valve stems were within .002" of each other. That's the correct way to do a valve job. Most shops that do most all Chevy stuff never look at that with a critical eye, because since the Chevys have adjustable rockers, they figure it doesn't matter, but it most certainly does.