When I did mine, I used a piece of 1/2" all thread about a foot long, it might have been 5/8"? and one of the "X" mounts that come with your motor and coincidently mounts to the firewall cause thats where the hole spacing comes from right?
then I used a couple nuts on the all thread to capture the X mount, and screwed the whole deal to the firewall, poof done deal