I think it is much more easier....
use the network-install iso form
http://cdimage.debian.org/mirror/cdimage/archive/3.1_r8/m68k/iso-cd/burn the iso on a cdrom and start the installation with your prefered Grafigboard.
the networkpath for the Installation is
http://archive.debian.org/debian sarge main contrib non-free
After basic-install you can try to do a Distupgrade with the correct sources in /etc/apt/sources.list See Post 1 !
should be
deb http://ftp.debian-ports.org/debian pool-m68k main
give it a try....
install aptitude with apt-get install aptitude
get the latest Kernel! see Post 1 install the kernel and copy it to your Amiga-Paritition
then do the upgrade
aptitude -f --with-recommends dist-upgrade
I think that´s it...