AC Delco Rapidfire #4 plugs in a slant six

BillGrissom said
Your engine has a solid head and should use the gaskets.

His engine has a "drool tube" head made between '63 and '74 and should not use the gaskets.

"Drool tube" is a clever euphemism for early slant six's (1962 - 1973?).

'60-'74.

The spark plugs mount inside removable aluminum tubes. The tubes seal against the head with a large rubber washer that is notorious for seeping oil in poorly maintained engines, hence the term. The washers are available and cheap, so change whenever the rubber hardens.

Those are called "spark plug tube seals", as distinct from spark plug gaskets (the metal items that need to be removed).