Right-O !
I think we may have found the vibration source... the TWIN vibration sources.
I took the Corvette out for another drive this evening and found that the vibration seemed a bit intermittent - seemed to be RPM related but also more pronounced at certain speeds... I also noticed that the source of the vibration seemed to move around... including coming from behind me !
Up on jack stands I found that middle of the exhaust pipes were bumping or in contact with the bottom of the car.
Tomorrow I will mess with it a bit and see if I can get some clearance there. If that changes the problem, hopefully... we are there !!!