I recently had an clacking noise on my 400. As part of troubleshooting to see if the lifters were spinning, I wiped the top of each pushrod. I used red nail polish and drew a small line at top of each pushrod. When the engine was started, the red line made it pretty easy to see if the pushrods were spinning. You can also check for how oil is spurting out the pushrod. As you know , this can get messy. So, using cut valve cover and other tips can help. I did troubleshooting with the valve covers off. I laid down some tinfoil all around the outside of each cylinder head and then placed some cloths around the tinfoil and even placed some cloths on inner fenders. As the oil heats up the oil splashing can get a worse.
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I had a few pushrods were spinning a bit slower and one that was quite a bit slower. One of my pushrods was also having an erratic oil spurt. I added about 16oz of Marvel Mystery Oil (MMO) and 6-8oz of Seafoam. After letting the engine run 2 or 3 times for 15-20m - the MMO and Seafoam definitely helped. All of my pushrods were turning very close to the same speed and the erratic oil spurt from the one pushrod was gone and my clacking noise was gone
