Slant six spark plugs and numbers
In brief....
There are two basic casting versions of the Slant six head. The most common head is on 1960-1978 (ish) and looks like the one in the bottom right of this photo. There will be an aluminum 'tube' that fits in the large round hole with the spark plug inside the tube. Spark plugs for this head will have the longer thread reach and a small metal gasket (remove for most slant applications).
The other style head was used from 1978(ish) to 1987. This head looks like the one in the upper left corner of the picture. Only the spark plug goes in these, no tubes or such. The spark plug for these heads has a tapered seat (NO gasket) and a short thread reach.
You cannot mix spark plugs between the two styles of heads.
In brief....
There are two basic casting versions of the Slant six head. The most common head is on 1960-1978 (ish) and looks like the one in the bottom right of this photo. There will be an aluminum 'tube' that fits in the large round hole with the spark plug inside the tube. Spark plugs for this head will have the longer thread reach and a small metal gasket (remove for most slant applications).
The other style head was used from 1978(ish) to 1987. This head looks like the one in the upper left corner of the picture. Only the spark plug goes in these, no tubes or such. The spark plug for these heads has a tapered seat (NO gasket) and a short thread reach.
You cannot mix spark plugs between the two styles of heads.