the DISTRO_PKG_SPECS in the ubuntu pups has grown & evolved from barry k's precise puppys DISTRO_PKG_SPECS.
i never started with a blank slate so to speak and have never stripped a puppy down further than the base fossapup.sfs.
maybe a way to go is right clicking on libs & binaries in the outputed xserver & xorg_base_new folder and using ListDD to show whats needed and make sure they are in the build.
if the iso then boots but doesn't get to X run checkdeps -system to see what's needed