Pete Aron - Dan Gurney or Phil Hill
Scott Stoddart - Jim Clark (obvious)
JP Sarti? This character has no direct parallel.
Nino Barlini - Lorenzo Bandini (obvious)
I think Aron was largely Phil Hill, but also Ginther, who had been with Ferrari, then BRM, then joined Honda and got back his fire and determination and won them their first GP.
The driving of Stoddart was Clark, and his relationship with Jeff Jordan was similar to Clark and Chapman; but his character and his dead World Champion brother were fictional.
Barlini had a similar name to Bandini, but was nothing like him as a person. Barlini was a young brash ex-motorcycle champion, and there were ex-motorcyclists in F1 (Behra, Surtees, Hocking, Anderson) but IIRC no Italians.
Sarti was indeed unlike any real driver; he was supposed to have been Champion in 1961 and 1964. Frankenheimer admitted he had got it wrong by killing off his most sympathetic character.
I think Frankenheimer wanted to use the banking at Monza because he had to come up with a crash that was even more spectacular than the one at Monaco with the two BRMs. Apparently he kept asking Phil Hill, his racing adviser (also stand-in driver, camera-car driver and occasional actor) how a car could happen to go over the banking until Hill admitted that if it ran over a piece of debris at just the wrong angle, it could happen. Maybe Frankenheimer knew about the crash of Behra at Avus in 1959.
Although the Monza banking hadn't been used for F1s after the first practise of 1962 and 1963, it was used for the 100k sportscar race as late as 1967, IIRC.