Mike,

From American Classic Truck Parts catalog: 1955-59 Valve Cover Decal

It is made to fit on the top side of the valve cover. Maybe someone else will know which side(s) of the valve cover and the location(s).

Most likely, the location(s) of the decal is shown in the 1955-59 Factory Assembly Manual. The location of the 54/55 valve cover decal is shown in the earlier FAM.

Tim