Valve float causes valvetrain separation. The cam has approximately 58 chances a second to knock a wheel of the bottom of a lifter when it's floating... My guess is the the valves floated and knocked the wheels off the bottom of some lifters. It takes a lot of effort to reliably spin a HR set up to 7,000 rpm or more. I can tell from the pictures this valvetrain is not designed to turn anywhere near that number.
Not sure why several folks seem hung up on the use of BBC rocker arms. One of our own members here has been successful using BBC rockers on his build for several years. If you start inquiring you will find most aftermarket rocker arms are based off a Chevy or Ford design and labeled Pontiac. A rocker arm has no idea what brand engine it is designed for.
The builder has already "cowboy'd up" and offered to repair the engine for the cost of parts before he even saw the issue. I'm betting he helps the OP a lot more than he should considering the situation.
Carry on!
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