washers are a bad idea. what are you going to rest them on, on the head? the fins? i doubt the fins are aligned with the face of the head and also you aren't going to find washers that are the right thickness.
another idea is to tighten up a sparkplug in and chuck that in a lathe. assuming they made the spark hole perpendicular to the head face. I know of a few heads where this is obviously not the case. but for ones that look close it would pay to check before you start making chips.