D+10 Dave the auto stores sell leak dye kits for about $45.00 dollars. They are easy to use. Pour the little bottle of dye in the motor, trans or what every is leaken and road test the car for about 10 miles or until fully warmed up. There is a light and glasses that will show you where the leak starts at and is located. Some valve cover leaks are from the intake that you only will find with the dye. What every sealer you use, the surfaces MUST be clean. The sealer should have a lite skin just starting to form on it and it needs a FULL 24 hours for it too setup. We all have different ways to build motors and different sealers are needed for each location. The GREY sealer is what we use where I work with full cure time allowed. Have not had any failures from doing intakes,
oil pans, and heads. I use Hi-Tac on head gaskets and Grey RTV on all other locations. I use the heavy cork valve covers with them glued to the cover, I check the rockers after racing. I reuse them for a couple of years or until them are crushed. Rear main seal is the normal problem for FE motors. If you have a rope seal, it will leak. I keep cardboard under the car for this. This is part of owning a cobra.
Rick L.