I don't think its the indycar, look closely at pic 2 and you'll see the fuel cap is right at the back, but on the suspected F1 machine its less than 40cm behind the rollbar, which is also slightly thiner than the first pic.
I don't know what sort of car pic 2 is so I so I don't know why I posted it, lol.
I can't see how a non flat-bottomed F1 car with a turbo engine could be anything but a Renault, Ferrari or BMW, and this doesn't seem to be any of those.
Almost all the CART teams around that time bought March chassis but they were often extensively modified, so that's still what I suspect.