My 69 Z has been sitting for about a year exceept for regular starting and checking everything it has not been driven. I took it out a couple of weeks ago and at around 60 mph there is substantial thumping or vibration. It is not a drivetrain issue because it only does it at 60 and has nothing to do with the rpm's. It has bias ply e70x15 wide ovals. Does this sound like wheel balance or flat spotted tires from sitting? Thanks

Most people forgot the flat spots that would develop in the bias plys , I remember in the winter it would take 1/2 hour of driving to get it out. If it wasnt there before it probably is the tires drive it some more on warm pavement. Good hour or so see what you get it goes away slow.