Hi Leighton,
My aircraft has the 400A without HSI and Figure H-31/page H-50 in the Garmin manual is for the 300B/400B/800B or the 300IFCS/400IFCS/800IFCS.
However, page G-3 in the same manual lists the 400A if it has the CA530FD autopilot computer which mine does. Bottom line, I think it's going to take at least one phone call from my avionics guy to Garmin.
I can tell you that the DAC GDC31 is STC'd and AML'd for the 400A in a 337G but there's no vertical guidance.
http://dacint.com/products/gdc31/