Going to race a 29er MTB in a 200 mile / 320 km gravel race. Bike preparation? I have a Specialized Camber Comp 29er full suspension MTB. I want to use this bike for the DK200 (a one day 200 mile / 320 kilometre gravel race through the Flint Hills of Kansas, USA) and would like to be reasonably competitive. I know, this guy is nuts.

That being said, I'm planning on gravel tires, bar end grips, possibly aero bars, bike sizing, and 'locking the suspensions'. 
I understand I will be sacrificing weight and efficiency/torque with the longer chain stay and suspension flex, along with gears that aren't quite tall enough on the downhills and long straights. However, I just do not like the feel of the gravel bikes. They're too stiff and bone jarring, and just don't like the general feel when I'm on them. I'm thinking over the long haul on my full suspension bike, that my suspension soaks up a lot of the potholes, ruts and rough roads will pay off in the long run. 

What else would you recommend that I do to my bike to properly prepare it? Any specific brand add-on pieces you'd recommend?

I am in good shape and currently outrun others on carbon gravel bikes on 45 mile / 72 kilometre rides on my stock MTB. I will be in excellent physical condition in 5 months when it comes race time. I do have some issues with my lower back and shoulders/neck area currently at a little over 45 miles / 72 kilometres, so needing to get more comfortable and in better shape. 

I know this was lengthy, but much appreciate some expert advice! Ride on!
Between mass and aerodynamics you would be expending a ton more energy on the MTB if the roads are in decent shape. Rather than roadify the MTB you might consider MTBifying a road bike.  Tires are the key, really large, supple ones to soak up the bumps, at an appropriate pressure. Perhaps aiming for tubular cx tires that you can run really low pressures on.