I installed the ISG into my 727 with no issues, what I wound up doing after the first install was to change it so that the MCU was sitting next to autopilot panel.  Add a hot button on my yoke and when I press it I get the MCU/autopilot at the top right of my screen.  The FMS I wound up replacing it with the Garmin.  

It flies perfect so now I can do RNAV SIDS/STARS.  The only thing that I have to figure out is how to get the CDI to work with it, so that when I want to track the course by hand instead of autopilot the CDI is responsive to the FMS and then when shooting the ILS I can revert it back.  I could probably add a switch that does NAV/GPS, unless I am missing something already there.