I only use grub, including on Slack, but it wasn't a slack package, and I never used one of Pats extra kernels, soooo ...
You will no doubt have to manage updates to menu.lst yourself.
If you saw some lilo output from the make, sounds like the Makefile is "tailored" by Pat - in which case you may have lilo back in the MBR. Try the following, and look for eyecatchers
dd if=/dev/hda count=1 | strings
Presumes hda as your boot disk, and "strings" is installed. I haven't run Slack for ages, so I don't know if those are valid presumptions.