Using the camera mounted on the motion platform with Rift doesn't work right. This software seems to work pretty well with iRacing. I haven't actually set up my motion platform to work with DCS yet, I haven't got enough free time.
https://github.com/matzman666/OpenVR-InputEmulator/
You fix one of your hand controllers on your chair near your head and the software subtracts the motion. Your virtual head still moves away from where it should be a bit, but way less than it would do without it.
The other option is to use no sensors at all. Doing this will fix your head in 3d space and you can only pitch, roll, and yaw it. Drift in the inertial sensor readings will also mean you need fairly frequent presses of the HMD center button.