Get yourself a borescope and look in each cyl through the spark plug hole. I'm guessing you have a rather aggressive camshaft and your pistons made contact with the valves and thus pushed the valve into the rocker while there was force coming upwards from the cam side. This caused nothing to want to give but the weakest link. The weakest link in this case was the needle bearings on the rocker. You'd be lucky if it's just the rockers that're bad. I've seen this happen on a 454 before and it needed new pistons.

 

Thanks for the info

 

Did you check your pushrods? Are any bent? They would probably bend before the valve stems. Have you done a compression(dry and wet) and a leakdown? This will tell the tale. I would change EVERY lifter. It looks like you've gone back several times only to find more lifters have failed. Change them all and be done with it. After that do a leakdown.