Fix for Cowl Shake and Body ?

Na... It's a convertible, I can't hear anything after driving for an hour on the interstate. When I drive with the top up, I hear all kinds of noises, but it is a 54 year old car not a modern plastic disposable car.

Just my 2 cents.

Many cars have spray on panel stiffener like that in our trunks on the quarters.

Anything to thicken the sheet metal will reduce the tinny sound you talk of.