A friend of mine purchased a 75 Grand Prix out of Saskatoon that "wasn't as described". The worst part is severe rot under the vinyl top. Replacing the complete roof may be the best plan of attack. Also the lower front fenders are bad, rear 1/4 panels need work and there's a hole in the drivers floor pan. Floor pans are available however not much else as far as sheetmetal is concerned. If you western Canada guys come across any 73-77's in the yards or know of any of these cars being parted out let me know. The car itself is kind of neat. Pretty much loaded with a 455 and positraction.
From looking at photos, the opera window shape appears to be different between the GP and the other 3 brands, but the rest of the roof stamping might be the same.
I parted one of these over 10 years ago, same color car 1975 Grand Prix 455 LJ . I will look in my shed for left over parts but I doubt I have much left. I was tempted to buy the one in Saskatoon for the engine but it was too pricey for me.