PCV could be a problem. I have a stock NOS one in it (for a 1972 440 engine or so), which might not work with the changes in my engine compared to stock. I will block it for a test and see if idle AFR will change significantly.
The lower the manifold the vacuum (relative to stock), the more the likely the PCV will be flowing additional air at idle.
I have only found published flow numbers for AMC.
Graphed here.The stock PCV valve is probably something you could work with. If you want to experiment, my suggestion is replace the stock valve with a fixed restriction. A starting point would be
Suggested restriction sizes for PCV test
