Another thing that might help If you decide to redo the gasket is put a SMALL bead of pematex sealant around the valve cover then set the gasket on that. That way the prematex holds the gasket in place while you flip it over to install. also make sure you dont slide the valve cover at all while installing it.

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86 300SDL. 250,xxx on #14 Head. One eye always on temp gauge.. Cruising towards 300K