Giuseppe Busso wrote the following in the foreword of Marcello Minerbi's book "Alfa Romeo-Zagato SZ TZ" (this is the English translation from the book; the the original Italian text may emphasize certain events differently):
The TZ1 is the result of the evolution of the concept of a 1500cc racing car that dates back to 1955 that is even before the SZ.
This concept first developed through collaboration between Alfa Romeo and Abarth-Boano on a box-type sheet metal chassis conceived by Abarth. The car was built as a roadster and tested, but due to technical problems which came to light during the tests, Alfa gradually lost interest in it. The idea was picked up again by Abarth at the start of 1958.
Abarth was then aiming at a coupé 1000 with Alfa mechanics and Bertone body, with a tubular chassis developed by Alfa, which was necessary to compensate for the lack of rigidity of the one in sheet metal.
This second Alfa-Abath-Bertone stage soon came up against difficulties - mainly caused by problems of cost.
However, the car was presented by Abarth at the 1958 Turin Motor Show and I remember it went down very well.
In an attempt to solve the problem of costs Abarth asked to be able to use the 1300 engine.
I recall that he didn't get an immediate "no" from Alfa so we collaborated with Abarth intensely up to the end of 1959.
In these two periods, the times I and Mario Colucci, my chassis expert, rushed backwards and forwards between Milan and Turin were countless. And, alas, one evening I came back alone - Colucci had been caught by Abarth.
It was undoubtedly his fortune because he soon became one of the people who counted most for Abarth, and he had a long and brilliant career there. The job of looking after the chassis passed immediately into the hands of another valuable designer of mine, Zava, another person who has been mentioned very little, I'm sorry to say, and the same goes for another engine designer, Villa.
After leaving Alfa Romeo, these two acted as valuable consultants for Autodelta and Euroracing.
I seem to remember that Abarth didn't stop at taking Colucci away from me. He was probably the one who tried the same trick with with other good workers of mine, Genta and Ceccherini.
This time I managed to ward off the blow by getting counterproposals for the two from Alfa Romeo.
I don't know whether by doing this I did the best thing, but I definetly kept at Alfa Romeo two valuable people.
In Busso’s book “Nel cuore dell Alfa”, he first writes about Abarth on page 90, which relates to the year 1958 (I leave translation to others with better knowledge of Italian) :
Abarth, dopo l’insuccesso del suo telaio per la 750 Competizione, ritorna all’ attacco proponendo che a disegnare il telaio questa volta fossimo noialtri. La proposta non era male perché un telaio che, a nostro avviso, non poteva essere che a traliccio di tubi, ci avebbre consentito, dopo la volta infelice della 6C 3000 PR
, di riprovarci. La cosa prese corpo rapidamente; a disegnare il traliccio toccó a Mario Colucci. Si cominciò in marzo; al momento Abarth non riuscì a ottenere dell’Alfa che l’autorizzazione per una cilindrata di 1000 cm³, ma lui non dubrivata di riuscire a smuovere la direzione per qualcosa di più se l’operazione, per la uale intendeva valersi della collaborazione do Bertone, avesse avuto il successo che lui si attendeva, L’Alfa Abarth riuscì addirittura a essere presentata al Salone di Torino in ottobre, con indubbio successo, ma una volta ancora il problemo dei soldi, che diede luogo a grossi bisticci fra Abarth e Bertone, soffocò nella culla quella bella creatura. Bilancio negative per tutti tranne che per Mario Colucci; Abarth, che aveva avuto modo in quell’occasione di apprezzare la grande capacità tecnica del mio allievo, non trovò di meglio che portarmelo via; bella fregatura per l’Alfa e per me, ma splendida occasione per il ragazzo di avviarsi su una strada che lo avebbre portato a notevoli successi.
While Busso doesn't directly answer the question that started this thread "Can anybody confirm the info, that the Disco Volante used an Abarth frame?", I believe its worthwhile to point out that Busso doesn’t talk about Abarth in the context of the Disco Volante at all but he does confirm that Alfa in 1955 experimented with a "box-type sheet metal chassis conceived by Abarth." The 1955 date contradicts the claim on p.42 of Sports Cars that "Disco Volante could not have been built on any other frame."
The way I read Busso's comments, I get the impression he doesn’t seem to express a lot of confidence regarding the quality or sophistication of frames designed by Abarth and that he believed Alfa's engineers (specifically Mario Colucci) were much better at frame design than Abarth.
BTW, I wish I would have read earlier Busso's comments about TZ protoypes going back to 1955 so that they could have been included in the Asardo thread that was deleted at some point.