AFAIK there is no need to rebuild kernel-headers nor (hopefully!) glibc.
the kernel-headers package simply copy headers from the kernel tree to /usr/include, this is why:
(1) There is no need for a SlackBuild for it.
(2) These headers (in /usr/include) won't be overridden if you install a new kernel-source, so there is no need to update them when you install a new kernel-source package.
Oh, and in any case the sanitized kernel headers should be there *before* you re-build glibc (which is hopefully not necessary in your case anyway) as noted in glibc.SlackBuid:
# NOTE!!! glibc needs to be built against the sanitized kernel headers,
# which will be installed under /usr/include by the kernel-headers package.
# Be sure the correct version of the headers package is installed BEFORE
# building glibc!
Long story short, just install your kernel 3.0.49, no need to change the kernel-headers nor glibc AFAIK.