Start with the config file from the regular slackware kernel, not the huge kernel.
For example, (I am using -current so you may have a different kernel-substitute your kernel version)
If you don't have any sources then install the kernel source.
cp /boot/config-generic-220.127.116.11 .config
Making the changes that I suggested: changing the <M> to <*> by using the spacebar
, In addition to General setup > (custom) Local version-append to kernel release
make && make modules_install
cp arch/x86_64/boot/bzImage /boot/vmlinuz-18.104.22.168-custom
cp System.map /boot/System.map-22.214.171.124-custom
cp .config /boot/config-126.96.36.199-custom
mkinitrd -c -k 188.8.131.52-custom -f ext4 -r /dev/sda8 -o /boot/initrd-184.108.40.206-custom.gz
Substituting whatever partition that slackware is on for /dev/sda8 and some other filesystem name if you aren't using ext4
Then update your menu.lst
If you have any questions feel free to ask.