Get some cast valve covers. If you must have the originals on iron heads, the best way is to smooth the rail on the head and glue the gasket to the cover using (sparingly) The Right Stuff. The cast rail on original heads is terrible. Narrow and filled with porosity. You have a 3/8 flange on the valve cover that tightens down to a 1/8"-3/16" rail. Over tightening can be a bad issue. The factory had those little boomerang and other pieces over the top of the valve cover rail for a reason.