You need some kind of washer under the valve cover screws. Many of them used kind of a skinney metal piece about an inch long. It was to help even the pressure on the cover and not bend it. I have tried everything on valve covers. I think what works best for me is to put it in dry and just watch it carefully when you tighten it down to make sure it doesn't slip. It seems like sometimes gasket sealer just makes them slick and they slip out easier when they warm up. Some gaskets have little ears that hook to the valve cover to help hold it in place. I can't if a 235 does or not.
Hope this helps
Jim