Let the smallest integer be \(n\).
Then the six smallest integers are
\[
n,n+1,n+2,n+3,n+4,n+5.
\]
Their sum is
\[
6n+15=321
\]
\[
6n=306
\]
\[
n=51.
\]
Thus the 12 integers are
\[
51,52,53,\ldots,62.
\]
The largest six integers are
\[
57,58,59,60,61,62.
\]
Their mean is
\[
\frac{57+58+59+60+61+62}{6}
\]
\[
=\frac{357}{6}
\]
\[
=59.5.
\]