I would go back to a stock balancer for you engine, when I balance a BBC ,4" stroke I will balance it external, anything over that in stroke I will balance it internal, only due to the amount of counter- weight on the ends of the crank. Just remember the external weights on the balancer & flywheel aid in balancing. The balancers main purpose is to control Harmonic torsional vibrations, think about what that crankshaft is doing on rpm changes, I always use a 8" on a boat and a smaller alum. 6" for drag racing application. Larger and heavier flywheels and balances store kinetic energy .