Based on my experiments with a first person vehicle mod, I think that vehicle_cameras.xtbl is the(or another) file that needs changing. Most of the vehicles have different camera positions in there.
Modifying the z_dist, as Uzi mentioned, would be where I'd start, but I don't think it will work out well.
I modified just a few vehicles, and setting the z_dist to 0.0 moved the camera into the cockpit, probably at the centre of the vehicle. And using negative values didn't work, they were treated as positive values, so in those cases, it wouldn't work.