[bigblockv6]

Both the 305 and 351 engines are internally balanced so there is no eccentric weight on the flywheel or flex plate as well as the front pulley, in fact there is no harmonic balancer or damper up front rather just a flanged hub that the pulley bolts to. There is an alignment procedure though. According to the factory GMC Truck Overhaul Manual on page 49 here is the procedure: Locate flywheel, flex plate, and reinforcing plate at crankshaft flange with bolt holes aligned. Be sure the letter "R" on flex plate and on reinforcing plate (fig 69) is toward rear as shown. Note: The letter "R" is located adjacent to offset hole in flex plate