Haven't tried it myself, but have an article saved somewhere. I definitely will do this on my Z during this rebuild. You say the metal is good, and I'm not doubting your assessment, but it can oxidize inside the frame as well, especially in the air chamber above the front wheels. This area is notorious for internal rust, and in a place which is difficult to see. Front frame rails also rust from the inside and can fold without much warning. Tapping with a drift punch may point out thin metal. There is also metal fatigue in otherwise sound-looking metal, which has flexed so much over the years to either become too soft or too brittle.
Are you sure you are not hearing movement between body panels or springs in their perches? Also the doors always were "tinny", and can be improved with panel sound deadening mats like Fatmax. I think that's a product name among others, and one that I have not used but will also definitely look into.
To respond to your question about strut tower bars, yes they will help, and yes a multipoint brace, say adding bracing from towers to upper firewall will help even more.
The foam won't help if the metal is bad, but will add strength, rigidity, and sound deadening at a slight addition of weight. Worth it I'd say,for a street car.
I owned a 70 Z when it was practically new, and they were not well insulated, much like most cars of the day, so they came with the problem. Today we expect our sports cars to have that solid "thnk" when we shut the door, etc, but look at all the sound deadening material they have, and the weight they have gained. Just make sure the car is solid and you may just have to learn to live with some of this idiosyncracy.
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