55+ vibration - Help

[garyd1961]

I'm a tire guy and all I can add is rotate your tires front to rear. If it's a tire problem the vibration should move from your seat to the steering wheel. I know you said you tried balancing your old tires and it still vibrated but I have seen more than one set of new tires that were so out of round you just couldn't balance them.
Oh, I forgot to say I had a 383 that someone left the bolt out of the balancer and started to wobble on the crank. Also double check to make sure you have the right flexplate for the motor, internally balanced vs externally balanced.