Keith is correct (as usual). The car is described, together with four other 8C 2900 Spiders, in "The 1938 Racing Season" on p. 266-285, with pictures of the July 13 accident (i.e. after the Mille Miglia, not before -- so, the caption in Autocar&Motor i soncorrect) on p. 277-278.
On p. 281-282, Simon Moore describes the accident as follows: "...on 13 July, one Spider was taken to Livorno to do some private practice alongside the new Tipo 158, the revamped Tipo 316 and a Tipo 312. The roads were closed for the occasion ahead of the Coppa Ciano that was to be held on 7 August. During the afternoon, when the single-seaters were not running, Pintacuda took the 2.9 out and, at the end of his third lap, left the road and hit a bridge parapet 500 metres before Antignano. There was substantial damage to the car which carried license plate MI 631. Looking at the photos of the crash scene shown on earlier pages, it was surprising they rebuilt the car -- maybe a new frame was used?"
During the Mille Miglia, license plate "631 MI" was registered to chassis number 412032 (engine 422019), mounted on a car with race number #148, driven by Villoresi/Siena. Whether or not this chassis number is correct remains unclear, as Simon Moore writes on p. 282-283: "Before the 2.9 story was published in Automobile Quarterly in 1973, the manuscript was sent to Cav. Fusi in Milano for comment. He identified the 1938 Mille Miglia cars as follows: Pintacuda 412030, Biondetti 412031, Farina 412032, Siena/Villoresi 412034, but why the cars would not have the correct registration plates is a mystery!"
Simon Moore further writes that probably was common practice to move around among the cars engines, registration numbers, and firewall chassis/engine plates -- but that bodies probably stayed on the frames, so he and others did much detective work trying to identify cars shown in pictures and match them to specific chassis numbers. And so it is possible that chassis 412032 may have carried license plate "638 MI" and participated with race number #143 in the 1938 Mille Miglia. More confusion exists based on the fact that pictures exist of two cars carrying race number #148 during Mille Miglia scrutineering.
According to the book, chassis 412032 was rebuilt in March 1940, possibly with the Spider body taken off 412033 that had been rebodied in 1938 as the Le Mans coupe (which is now in the Alfa museum), and sold in 1948 to Romano in Brescia (who drove it in the 1948 Mille Miglia) with a certificate of origin dated July 1947. As 412032 seems to be the only one of the five cars that was rebuilt, it is most likely the car that was involved in the Livorno accident.
The car was later sold to Studer who raced it in Switzerland, and severly crashed the car in the Erlen Grand Prix near Zurich in 1951. Studer then used at least some of the remains of 412032 for a considerably modified Special built in the early 1950s, and sold it in the early 1960s to a Mr. Eckert, who in turn sold it to Schlumpf in Mulhouse, were it is now on display in the National Museum "Cité de l’Automobile" (the picture below is from this page, where it can be seen much bigger).