This has been the bane of my existence for the last 3 months with my 408... I too just put on some trickflows because of the same problem with a different set of heads.. except my problem went boom real quick.
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I tore the top half of the engine down since I was replacing the heads anyhow, also removed the oil pan and pump. Cleaned and double checked everything then put it all back together with the new heads. I have a baffled oil pan so it tend to trap stuff even with oil changes, it was best to remove it and clean. Do yourself a favor and use studs for the rocker shafts.
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