The reason for the solid wrist pins is some 351 engines used the forged steel crankshaft from the 401 so to make up for the weight difference of the smaller and lighter 351 pistons solid wrist pins were used. As far as vibration dampners really out of the gas V6 engines only the 478's had them, the 379 and 432 just had eccentric external weight balancers since they basically used a 478 type crankshaft. The 401's basically used the same setup as the 305 and 351 just a flanged hub for the pulley to bolt on to. Here's a pic of both types of wrist pins
