I have a Mac Pro 1,1 that is currently running 8.11.0 using a Radeon 570 video
card.

I am using refit boot selector to boot the debian 9.6.0 amd64-i386 netinst.iso

    Also installed on separation partitions: OSX Snow Leopard. This hosts the
refit program.

What works: boot the *.iso and installing i386 on separate boot/swap/root
partitions.

What doesn't work: boot the *.iso and select the amd64 CPU option. The
installer program just hangs at the first spash screen.

I tested the install dvd on another system with UEFI (a modern HP 470 G4). It
boots in UEFI mode and started to actually run the installer.

An alternate was tried. dist-upgrade on a running amd64 system. It does not
work for me. Here is the procedure I used:
        a) install a sacrificial 8.11.0 amd64 system on the same set of
partitions targeted for the 9.6.0 install
        b) modify the /etc/apt/sources.list to use stretch
                doing a dist-upgrade; shut down and reboot production system.
                on the production 8.11 system , created a grub boot stanza in
                          40_custom for the 9.6.0 dist-upgrade partition set
                make sure nomodeset is present
                update-grab
                then boot the production system using the 40_custom boot stanza
for the 9.6.0 dist-upgrade target partition set:
                         just shows loading vmzlinux and hangs

I have looked at the xorg modules installed before and after the dist-upgrade-
they all seem to be ATI or Radeon related.

My tentative conclusion is that something about the radeon video card and the
          amd64 option Mac Pro 1,1 is not compatible with Stretch.
    a)The radeon video card works fine with Jessie installer dvd and
       8.11.0 on the installed system.
    b)The radeon video card works fine on Stretch i386 installed system.