The pushrods may have failed at high RPM when the valves started to float. I've broken them just cruising around town, but I blame it on the high-performance Chevy 454 LS6 valve springs and some old (fatigued) rocker arms..
Might check to make sure the cam hasn't been scored or damaged before you spend the money on new lifters. You may need to get a cam to go with the new lifters.
What the chunk of metal you're describing is.. I dunno. Are you sure it isn't a lifter itself?
BTW, then run a LOT better when you get all 8 firing.. ;-)