Going to race a 29er MTB in a 200 mile / 320 km gravel race. Bike preparation?
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.