[Rick Parker]

With a dual plane manifold it may be somewhat difficult to see/get around the curves. Have you looked thouroughly around the top of the engine, footboxes, etc? Near the distributor base, front cover, water pump, etc. Try a mirror and flashlight and look inside the runners. It is possible that it COULD end up in the cylinder (yuck) if it went down a runner with an open valve (NOT GOOD). Don't start it until you find the missing washer OR verify where it isn't (if you know what I mean) this is unfortunate.
Good Luck