Answer by taking a quote from the following article:

Fangio's grandfather, Giuseppe Fangio, emigrated to Buenos Aires from Italy in 1887. Giuseppe managed to buy his own farm near Balcarce within three years by making charcoal from tree branches. His father, Loreto, emigrated to Argentina from the small central Italian town of Castiglione Messer Marino in the Chieti province of the Abruzzo region. His mother, Herminia Deramo, was from Tornareccio, slightly to the north.

After finishing his military service, Fangio opened his own garage and raced in local events. He began his racing career in Argentina in 1934, driving a 1929 Ford Model A, which he had rebuilt. These local events were unlike anything in Europe or North America, they were long-distance races held on mostly dirt roads up and down South America. During his time racing in Argentina, he drove Chevrolet cars and was Argentine National Champion in 1940 and 1941. One particular race, which he won in 1940, the Gran Premio del Norte, was almost 10,000 km long. This race started in Buenos Aires and ran up through the Andes to Lima, Peru and back again, taking nearly two weeks with stages held each day. Following many successes driving mainly modified American stock cars; he was funded by the Argentine Automobile Club and the Argentine government and sent to Europe in 1948 to continue his career.  In the Tourism Highway category, Fangio participated in his first race between 18 and 30 October 1938 as the co-pilot of Luis Finocchietti. Despite not winning the Argentine Road Grand Prix, Fangio drove most of the way and qualified in seventh place. In November of that year, he entered the "400 km of Tres Arroyos ", but it was suspended due to a fatal accident.  In 1939, the circuit was in Forest, which conformed well with his last involvement with a Ford V8. With Hector Tieri as his partner, they led Turismo Carretera that year with a Chevrolet, competing for the Argentine Grand Prix. Suspended by a strong rain and resumed in Cordoba, he managed their first stage victory, winning the fourth stage from Catamarca to San Juan. In October, after 9500 km of competition in Argentina, Bolivia and Peru, he won his first race in Turismo Carretera, the Grand Prix International North. He became the first TC Argentine Champion to have driven a Chevrolet.  In 1941, he beat Oscar Galvez in the Grand Prix Getulio Vargas in Brazil. For the second time, Fangio was crowned champion of Argentine TC. In 1942, he ended South Grand Prix in tenth place in accordance with the general classification. In April he won the race "Mar y Sierras" and had to suspend the mechanical activity due to the start of World War II.  In 1946, after a brief period of inactivity, Fangio returned to racing with two races in Moron and Tandil driving a Ford T. In February 1947, Fangio competed at National Mechanics (MN) in the circuit Retirement, and on 1 March, he started the race for Rosario City Award. Subsequently, Fangio triumphed in the circuit 'Double Back Window' Race.

Were AMerican cars used?
In 1939, the circuit was in Forest, which conformed well with his last involvement with a Ford V8.