I was looking into the 660 to replace my venerable 296 and had the same question. Maybe Airgizmo can tell? They should be in the works already to make a dock for it.
Talked to the folk at Airgizmo they say they should have a panel doc for the Garmin 660 finished and available for shipping in 1 month AND it will fit into the same space as the old X96 Airgizmo panel doc.
Seriously considering buying the 660/Airgizmo setup to replace my trusty but small screened old panel mounted 496 and adding a GDL 39 3D for AHRS on the synthetic vis. Seems like a lot of good features and kit for around $2000. I see the 660 can be ordered with a "bare wire " accessory but wonder how much bollocking about it is to replicate the comms push connectivity, existing BNC antenna cable etc, and I assume I would have to update my wing mounted Garmin 496 antenna for a waas capable one also.
If I'm thinking correctly the GDL 39 will give me dual band adsb IN and if I do a software upgrade to my existing Garmin 330E transponder to a ES version I can use the GDL 39 for the adsb OUT positioning source giving me the whole shooting match of adsb, geo referenced IAP's, synthetic vision and back up AHRS to the old vacuum system. My dream panel setup. Am I on the right track or am I missing anything?
Project for over the summer me thinks !