Would an '83 pickup be the correct flywheel. Definite large center hole for crank register. Definite cast crank motor but...... might have been a hydraulic cam motor. (Had an odd shaped valve cover in bed). Not sure if that has an effect on engine balancing.
All slant sixes are internally balanced, cast or forged crank. The flywheel size depends on the transmission and clutch size. The NP435 for example used the large (172? tooth) flywheel.
