On a 64-bit system you can use the "grub" package from the 32-bit version of Slackware.
You can run "grub" using Alien Bob's multilib packages or boot "grub" from a CD or floppy to use the "native" command mode.
Even without the multilib packages "grub" will boot a 64-bit kernel. Without multilib you have to install "grub" using a grub boot CD or floppy. Use "info grub" to find out how to create the grub boot CD or floppy disk.
To boot from the hard disk using "grub", copy the files from "/usr/lib/grub/i386-pc" to "/boot/grub" and then create the file "/boot/grub/menu.lst" with your boot menu configuration. The "/boot/grub" directory can be in the same partition as your 64-bit kernel. You don't need a separate partition for "grub".
Unfortunately you can't copy or edit files using the "grub" command mode. You will have to copy the files to "/boot/grub" and create "menu.lst" using the Slackware setup CD or some other OS. However, if you boot a "grub" CD or floppy you can type in commands to boot Linux from the hard disk.
kernel /boot/vmlinuz root=/dev/sda1 ro
You can also put a "menu.lst" file on the boot CD or floppy with a menu entry for booting Linux on the hard disk.
After the files are copied to the hard disk, boot from the "grub" CD or floppy and press "C" to go into "grub" command mode.
You type in two commands to install grub. For example:
The above commands are for installing "grub" to the Master Boot Record of the first hard disk when the "/boot/grub" directory is in the first primary partition on the first hard disk.
To install "grub" to the Partition Boot Sector of the first partition instead of the MBR use these commands.
The nice thing about "grub" is that you don't have to re-install "grub" after you change the "menu.lst" file or the Linux kernel files. Usually you just install "grub" once and then edit "menu.lst" as much as you like to change the boot menu. Grub will even work with soft links like "/boot/vmlinuz" and use whatever the links currently point to.
If you have an unusual boot device such as "fake RAID" that LILO can't handle, you may still be able to install "grub" from "native" command mode by booting the grub boot CD or floppy. You also need the required root device and file-system drivers in the kernel or "initrd" so that the Linux kernel can mount the root file-system. I've used either "dmraid" or "mdadm" on Slackware 64-bit with "grub" to boot the kernel and "initrd".