Windscreen Interchangeability between 65 Pontiac GP and 65 Buik GS
Very doubtful that they would be interchangeable. The 65 GP was built on a full size GM platform and the 65 Buick GS was a mid size platform like a Lemans or Chevelle. Try full size Pontiacs like Catalinas and Bonnevillles or other GM's like Impalas, Lesabres, or Deltas 88/98's.
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