Blown Head Gasket or Cracked Block

mopar340dave said

good stuff here:

The standard spark plug for the slant six is a N-14Y. If the end plugs tend to foul during warmup,
the cylinders #1 and #6 may want a hotter plug such as a N-18Y. For high performance
applications, the N-10Y or N-12Y should be adequate.]

Click to expand...

Those numbers are for Champion plugs, which in my opinion are garbage. I have much better results with Autolite's or NGK plugs. If using Autolites, check the box, to see where they were made. Autolite is shifting a lot of their production to China, and I wouldn't use them if made in China.