I had the problem of
oil getting into my coolant on my old 390. I found a fix from gessard (sp) machine shop. There was a write up with detailed instructions. The fix was to sleeve the
oil galley that comes from the cam to the heads. There is one on each side. Suggested using a roll pin or hollow push rod tube. I used a stainless piece of tubing and epoxied it in with JB Weld. I used a q-tip and applied the epoxy sparingly. You want to be sure you don't put the sleeve in so deep it contacts the cam. I also had iron heads and had them resurfaced where they meet the head gasket. Machinist said the surface was like a washboard. The problem went away so I'm not sure what fixed the problem. My tube installation, new head gasket or resurfacing of the heads ?? The engine stayed in the car and didn't have to be removed. I was purging
oil from the coolant system for a long time.
Good luck.
John