as an extension post to the "diagnosing vibration issues" article, I thought I would do a separate one about checking to see if the flex plate is on right without disassembly of the engine or removal of trans. a large percentage of the time that a customer has a vibration issue, the flex plate is on wrong or the balancer has "walked" from the crank bolt being left loose.
all nailheads are externally balanced. the flex plate will bolt on 6 different ways, unlike many other engines that don't have a symmetrical bolt pattern. the 322 and 264 engines, however, cannot have the flex plate bolted on wrong because those engines have a guide pin on the back of the crank to insure it get indexed no matter what. starting in '57, however, the guide pin was replaced with a simple index dimple indentation on the back of the crank mounting flange that is supposed to correlate to the index hole between the bolt holes on the flex plate itself. its common that people don't know the flex plate is counter weighted and/or they assume that if the bolt holes line up, then its on the right way if its off, even just one bolt, the engine is guaranteed to vibrate and shake. also, if you have a 364 flex plate on a 401 or visa versa, it'll also shake, as the 364s have less counter weight on the flex plate and balancer.
with the flex plate cover off, the offset "lobe" of the crank flange should line up with the center of the flex plate counterweight. once you know what to look for, its really easy to spot.
shown below is the dynaflow and 400/300, 401 flex plates. not shown is the 364 flex plate which looks just like the 401 except the heavy end doesn't have the notches in it
take note and compare the location of the index hole with the back of the crank and the correlation to the counter weight
shown below is an example of the flex plate being bolted on wrong
