at least in the 2.6 kernel series, it's supposed to NOT being an elegant solution to use straight kernel headers
its not, but 2.6.13 is the only version of 2.6.13 series available (as far as stable
is concerned). So its the only headers available.
However, try this.
The initial kernel headers and installation sets the lowest possible kernel version to use. If you use the kernel headers and the kernel of 18.104.22.168 to build your system. Once the system is built, turn around and apply the 2.6.13 patch file to your kernel (best results, leave the kernel vanilla before doing this). Once that is done, you should hopefully be able to use 2.6.13 and patches thereof.