I haven't been around much guys - sorry about no updates.........
Anyway, I was able to squeeze the HCI under the stock (non-modified) PDM mount, within the battery box - only after I cut those large connectors off the PDM and Harness. I choose to hardwire the PDM / Harness / HCI together as I had places to go, people to do..;-)
The above procedure worked fine (other than being a f*ck'd up thing to have to do - hardwired and non-removable without wire cutters of course). Everything worked as it should. That is, until after the successful test run that a choose to put an enhanced light (nameless) on my ride at which point I quickly discovered that the stock light worked just fine and that I should have left well enough alone....

(Cool video of my dually running over a HID kit is a separate topic)
To summarize, make sure your PDM kit has the newer revised small connectors on it (I haven't personally seen these yet btw); I can tell you that if you have those large connectors and try to install the way I described above, you won't be installing your side covers. If your not fond of having side covers - it will work. Regardless, I do not recommend installing a HID with the PDM; I base this on my personal experience only. -~700$ - check.
For the Air Ride, I simply installed a toggle switch under the left side cover to activate the compressor (on/off - no purge in my case, but that is easy too if you want it).
Hope this information helps the forum - ride well....
Peace,
/jim