My first post, hopefully it'll be of some use.
The only problem which I noticed is that you refer to the second linux partition incorrectly in the grub configuration file. Grub starts counting at 0 but GNU/Linux starts with 1. I believe you need to use a line like:

blah blah blah
title Slackware Linux
root(hd0,8) <-- 9 changed to 8
kernel /boot/vmlinuz ro
blah blah

I believe that should fix it if you haven't already