Are you running a 3" harness? 2.5" shoulder belts help a lot. I don't like putting 3" shoulder harnesses in endurance cars. Awfully easy when in the course of doing a driver change to have a shoulder belt slip off w/o being noticed in the commotion of a pit stop. A 3hr stint is a lot of jostling around. 3" belts seem to slip off sometimes. I imagine your carcass moves around as much, if not a whole lot more in that rig than a sports car drivers does. (except at Sebring)
Don't know if 2.5" is legal in the OR world. It is in FIA, SCCA, NASA. If you have to use 3", make the most of the graphic below.
Another thing to check is the angle the harness leaves (heading aft) the hans. It should be 1-2" below horizontal. That can be tough to uniformly achieve if you have multiple drivers of differing torso lengths. Key harness config measurements below:
