I had trouble getting the Chevy washers to work on a 300. They hit the port wall before they seat on the mounting tab.
Yeah, I always wondered why the Ford thick washers were only "almost big enough". I think they have been responsible for damaging many aluminum aftermarket manifolds as they just don't offer enough bite on the aluminum.
I have made washers out of flat plate in the past. The stock washers are not heat treated as I recall.
