Replaced valve cover gaskets on the 400 today. I was starting to smoke the exhaust pipe after 5 minutes of run time. It went better than I thought. The old gaskets(about 12 years old) were dry and brittle.
I tried something different this time - I put Permatex gasket sealant on the valve cover, then put the gasket on the cover and put the bolts through the holes from the inside to hold the gasket in place until the sealant got tacky. I then pulled the bolts and installed the cover, with only one minor adjustment to the gasket to fit the bolts through.