if running headers,dont over look some of the factory welds and the 4 into 1 colletor
I have found a bunch of header will leak there..i only use cheap ones though
also look them over good for cracks in the "whole length" of each tube
with most flat mannys to head seal I use the copper rtv over the gaskets that come in whole engine kits (let cure over night)
also do this on bad/warped header flanges to head with header gaskets ( results will vary )
now mine does have a heck of a clatter that is the lifters till the oil hits it some times on start up...but can hear it clearly...
after running you only hear it in the cab
been chasing it for a long time on mine with some worn out engines over the yrs