I wouldn't rush to modify the flywheel on a six cylinder diesel - mainly because the inertia of the whole system is tuned in conjunction with the vibration damper on the nose of the crank to prevent the buildup of damaging torsional vibration. If you change the rotary inertia of the system, you change the resonant frequency of the vibration, and you may move it to a frequency where the vibration damper no longer works to reduce the amplitude of vibration. As the vibration is torsional, and high frequency, you won't feel it, but it causes damage nevertheless. The stress resulting from this torsional vibration is responsible for snapping crankshafts, and this is a big problem in diesel engine development - meddle at your peril.