Finding the correct spark plugs.
You should carefully inspect the area where the spark plug seats against the head to see if it’s flat(to use a gasket), or tapered(tapered seat plug) no gasket.
The reason I mention that is because it appears that Champion plug you have there isn’t supposed to use a gasket.
Edit-
The digging around I did showed a 67-74 318HD used tapered seat 18mm plugs.
To be used in a non-HD application, I’d suggest some NGK WR5’s
You should carefully inspect the area where the spark plug seats against the head to see if it’s flat(to use a gasket), or tapered(tapered seat plug) no gasket.
The reason I mention that is because it appears that Champion plug you have there isn’t supposed to use a gasket.
Edit-
The digging around I did showed a 67-74 318HD used tapered seat 18mm plugs.
To be used in a non-HD application, I’d suggest some NGK WR5’s