GIUSEPPE VERDI ... 435Hz, 432Hz, 256Hz
When we speak about C4=256Hz, then ONLY IF Pythagorean temperament (or a variation on it, such as Renold I) is used, we find A4 indeed at 432Hz.
There is a "problem" though with stating that Verdi is a "432-composer" based on his "preference" for C4=256Hz ... and that is that in Verdi's time the Pythagorean temperament was no longer used, but various other temperaments (Equal Tone Temperament and other).
When using C4=256Hz as concert pitch A4 might NOT be 432Hz, but could result in A4 ranging between 428Hz and 430.5Hz, depending on the temperament. If you list the Pythagorean Temperament (and variants) as Just Intonation (5-limit), then the range would be extended to 432Hz and possibly even higher.
Using the "Scientific Pitch" as argumentation thus does NOT seem to be the "best evidence" to be put on the table, specially not with proper specifications about the used temperament.
A better validation for the claim though is that Verdi seems to have supported the French national pitch, the "Diapason Normal" of 435Hz. The Conservatory of Milan (Italy) has a letter written by Verdi in their collection (found by Maestro Roberto Gorini Falco, who's daughter Liliana Gorini provided the image below):
The images above unfortunately ain't photocopies of the letter itself, but text as presented was provided by the Conservatory. I hope some day a proper photocopy of the actual letter itself will be made available.
If you (the reader) by any chance have a photocopy of the letter itself in your possession, then you would do me a great pleasure providing a copy. Below the translation of the text (online translation):
Genova, 10 Febbraio 1884.
“Fin da quando venne adottato in Francia il diapason normale, io consigliai venisse seguito l'esempio anche da noi; e domandai formalmente alle orchestre di diverse città d’Italia, fra le altre quella della Scala, di abbassare il corista (diapason) e di uniformarsi al normale francese. Se la Commissione musicale istituita dal nostro Governo crede, per esigenze matematiche, di ridurre le 870* vibrazioni del corista francese in 846*, la differenza è così piccola, quasi impercettibile all'orecchio, ch'io aderisco di buon grado. Sarebbe grave, gravissimo errore, adottare come viene da Roma proposto un diapason di 900* vibrazioni.
Genoa, 10 February, 1884.
"Ever since the "Diapason Normal" was adopted in France, I advised we also follow it suits us; and formally asked the orchestras of several cities in Italy, among others of that the scale, to lower the chorister (pitch) and to conform with the French "Diapason Normal". If the music commission established by our government believes, for mathematical requirements, to reduce the 870* vibrations of the French chorister to 846*, the difference is so small, almost imperceptible to the ear, I adhere willingly. It would be bad, a very bad mistake, to adopt a 900* tuning fork vibration as proposed in Rome.
* 870 -8va = 435, 846 -8va = 432, 900 -8va = 450
Verdi seems to have supported the "Diapason Normal" of 435Hz and did not object against an even lower pitch of 432Hz. This makes Verdi so far the ONLY composer that has actually mentioned 432Hz (as far as proper historical evidence goes).
You might wonder, what about other composers and musicians?
You might have heard other names mentioned along side Verdi, like Mozart or Bach, but also contemporary artists as Jimi Hendrix, Bob Marley, Pink Floyd and Prince ... ? Well, if you like to know fact from fiction, I suggest you read the Roel's World article "Myth: Mozart, Jimi Hendrix, Bob Marley, Prince ... 432Hz artists?"