340 is knocking

id check the rockers first, seeing as i have seen this on a motor me and my dads friend put a set of eddy heads on. the pushrods were 3/8's race stuff from the 80's and not the 5/16's like the holes were drilled for i think. so they were binding up, and when he thought he had the rockers adjusted properly, they were bound up and chattering.
co
id replace the header gasket, and pull the valve cover off and give them a look. its usually the simple things that will mess you up is what ive noticed.

hope this helps!