I think you should either specify the version you want to install (if I remember right it should be possible by adding
=4.14.79 to the package name you are installing via apt-get, but the string should match exactly the version, it may be numbered slightly differently), or update the kernel image itself to match the headers package.
The entire script to build our custom kernel build is available here: https://github.com/BlokasLabs/rpi-kernel-rt/blob/master/build.sh - it is meant to be run on a desktop Linux for faster build time, but you can replicate the build on Raspberry Pi itself by skipping lines 8 and 9, and use
-j4 for make instead of
We don’t yet have our own solution like rpi-source for getting the right kernel source for the current kernel version, but at the time of writing, you can still get the right kernel code by doing:
git clone --depth 1 --branch rpi-4.14.y-rt https://github.com/raspberrypi/linux, commit hash 22bb67b8e2e809d0bb6d435c1d20b409861794d2