Why do I keep breaking rocker arms?

So I finally took my rebuilt 225 (1700 miles on rebuild) with its 3-speed manual transmission and big cam in to my mechanic to have it set up with one of @halifaxhops' hotrod distributors and during the tuneup something sounded off and when they checked the valves they found a broken rocker arm, the second since the rebuild (the first happened during the break-in). Mechanic says I must be revving too hard. I've got a tach so I know I tend to cruise all three gears in the high 2000s and usually shift up in 3000-4000 zone depending on conditions and level of friskiness. I thought slants could take a pounding. Am I doing something wrong? I s my mechanic full of ****?