1974 Dodge Dart Magnum Swap

Not saying anybody is a jackass. I have had 2 5.9's completely rebuilt. Machine work only. One for my 99 5.9 R/T Dakota, a spare and the 408. Bought the 408 used and was told everything is perfect. Knew the mechanic the built it personally, Jim Boostin, and I trusted his work. However, I had the 408 rebuilt be another machine shop and they found one piston with a broken ring land. Replaced the piston set. Jeff died of cancer 4 years ago. Got the 408 from his son. My point is, whenever you get a used engine, the unknowns can cause much grief. Rebuilt or not, and now you are experiencing it.
Just trying to look outside the box. Good luck with your project, hope you find the problem.