Maybe someone else can help you with a source for the elongated washers that were used on 54-62 235/261 valve covers (you said you had four "bolts"), but they look a little like
washers in this set. The difference is that the elongated 235/261 washers were symmetrical.
Have you checked the bottom edge on the valve cover, especially near the bolts holes, to be sure that the edge is not deformed? It is quite common for the area right around a bolt hole to be deformed downward - due to a heavy hand in tightening the bolts (to try to stop a leak, usually).
