The movie's shooting schedule probably didn't leave Hutcherson-Pagan or Banjo Matthews enough time to fabricate enough NASCAR-spec Tbirds. (their regular NASCAR customers would've just reskinned their 1982 T-birds with fresh metal) Since NASCAR mandated the new 110" wheelbase across the Winston Cup cars regardless of factory origin (GM coupes were on a 108" wheelbase. The '83 T-bird was on a 104.2", according to wikipedia.), a street 'bird would be a bit shorter between the wheels.
Something else to mention--I tried a parts swap of the Thunderbird grills between the Monogram NASCAR kits and the Pro Stock kits. They aren't the same. One of them had more vertical curvature than the other, but I forgot which one.
