Just did this on my FC40, went really well actually. You'll need 3 items. The Zenmuse of your choice H3/H4, the Phantom 1 upgrade board Part #25, and the Mounting Adapter Part #3 (includes slightly longer landing gear).
Install the upgrade board per the DJI instructions, takes a hot soldering iron and lots of pictures just in case. Then attach the adapter plate to the underside of the shell. The H3/H4 attaches to the adapter plate, the ribbon cable attaches straight to the upgrade board. You will NOT need the GCU with this setup, keep it in the Zenmuse box for spare parts. You will still have proper video outputs via the plugs on the upgrade board and if you get lucky with a V3 board, you'll also have a CAN-BUS plug for iOSD later.
The Nazam software on my Mac sees the gimbal, via the regular onboard USB plug, and allowed me to set the lever up on the back of my controller. I have not attached any video to the system yet, but I plan to in the coming weeks, just have to decide what hardware I want to use.
Took me about 2 hours to do the total install, but your mileage may vary.
--B