Although this doesn't answer your specific question, it is supplemental information that may benefit. Generally speaking, manifolds are cast whereas headers are tubular steel. With most headers, replacing spark plugs can require you to be a contortionist. Headers often shroud access to at least some of the spark plugs making them accessible only from underneath. Some configurations will require you to dimple one or more pipes just to install a spark plug socket. There are special sockets available just for this reason. In addition, heat resistant socks are highly recommended to increase wire and boot longevity due to the intense heat radiated by the pipes right off the heads.
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