Blown Head Gasket or Cracked Block

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.

http://www.slantsix.org/articles/Slant Six Racing Manual.pdf