Weird vibration

funny story sorta. back decades ago, I went back to hometown in sw Ga to visit and as always took the trailer, and found a car. 68 gtx, nice driver, original 440 at. ran great. boght it. well I stopped off at car buddy on way back home. he wanted the X for oa o drag car. he takes it out that night late, opens it up all the way. faster he goe the more he feels a vibration maybe from the engine?
next day, him being a racer, he takes the engine apart ( as they all love to do), and he finds 5 standard 440 rods and 1 heavy "sixpack" rod!!!!!! ha