JRS is right, it was the 643 that was deemed to be truck-like and those comments (no doubt amongst other issues) got Prost the boot.
Was it Arnoux who got fired by Alfa Romeo over comments about their engines in the 80s? I'd need to look it up to be sure but I think it was.
EDIT
Almost right, I knew it was something to do with Arnoux and Alfa - (from Wikipedia)
"The Ligier JS29 was a Formula One car designed by Michel Tétu and Michel Beaujon for the Ligier team for use in the 1987 Formula One season. The JS29 was originally to be powered by a brand new Alfa-Romeo 415T 4cyl engine, however, after pre-season testing lead driver René Arnoux compared the engine to used food. This gave an excuse for Alfa's parent company Fiat to pull the plug on the project. It was reported at the time that Fiat did not want Alfa Romeo involved in Formula One as a competitor to Ferrari."