Kernel source would be in /slackware64-14.0/slackware64/k/.
Why roll back so far to 2.6? You should be able to utilize the .config then roll your own from source;
Slackbook is rather dated. You can still use for references but be aware that latest kernel stable is now 3.10 from https://www.kernel.org/
Slackbook beta: http://slackbook.org/beta/
is still a work in progress. You can use the beta along with the old Slackbook to help with work on Slackware. Slackware® Basics
is another useful source.
BTW, slackware64-14.0/kernels/ is the stable Slackware 14.0 release(huge.s default kernel) ;