What I recall is....when I first got the car to town I took it to R&A Motorsports a very renowned Mustang restorer and performance shop.
When the Fox Body Mustang guy lifted the hood to look around he pointed at the valve covers and said that I had the wrong valve cover gaskets. I knew the car had had a valve job so I assume it was from reinstalling the covers. Anyway they were changed at that time to the correct gaskets.