We need to start with eliminating the head gasket for sure. This is the most likely cause and with the amount of work that has been done involving the heads, we need to make sure they are not the cause as they would be the first thing I would think of. Here is a guide that helps with this:
https://www.2carpros.com/articles/head-gasket-blown-test
The other likely causes are leaking oil cooler, which I don't believe the 3.0L had one, and cracks in the head and block. It is unlikely that the manifold not sealing is causing this because you don't have coolant going through the intake.
I would suggest starting with this test on the head gasket and then go from there. Let me know if you have questions. Thanks
Saturday, June 27th, 2020 AT 8:52 AM