I just picked up a whole truck on a trade from a previous deal last summer. It's a '76 K20 with a 350/TH350/203. Originally it had a SB400 according to the grille emblem. When I drove it home yesterday it felt like it had a vibration in the motor. I figured it needed a major tune up. Then I thought, well, someone swapped in the 350... I wonder if they knew enough to put a 350 flexplate on it and not the original 400 externally balanced one. I look under, and sure enough, a 400 balance weighted flexplate. This makes several rigs Ive gotten that had improper flywheels on them. The last one was a Crew Cab that had a 454 flywheel on a 350. Hopefully the bearings havent been hurt too badly in this motor.