I wonder if the camshaft might have gotten damaged during the break in period? You need special greese when installing the camshaft, and when you first start the engine for the first time you can not let it idle. The cam needs to do a quick break in and oil lube when you first start it. Some say to keep 2000rpms for 2-5 minutes and that breaks in the push rods and cam. They also say if you let it idle during the break in time that some camshafts will go flat (bad). Older chevys camshafts were made with cheap metal and was famous for people ruinin camshafts on new engines. haha