Have the rear main seal replaced while having the clutch redone. It's a good chance that it's leaking now.
Your oil pressure seems to be normal. A real gauge is the only way to know for sure, but what you describe is similar to what I get. My oil pressure needle normally sits at the point where it is perfectly horizontal and dips below that every now and then. I have a high volume oil pump in a 351.
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351W 89 Mustang GT Convertible