The door tag and Marti report will be your true guides, but I haven't seen any 69 Mach 1's with the Mustang script on the front fenders. Unless of course, the fenders were replaced. I'm no expert, but that in my mind, that would warrant some additional investigation.
I would guess it is a GT
Current piles of rust 1965: New yorker***1968: GTO convert, Lemans convert Porche 911***1969: MustangGT convert, Mach1 S code Mach1, Mach1 CobraJet R code GTO convert, Firbird Convert 1970 Boss 302***1972: Charger***1999:f250 CNG truck***2011:nissan Altima ---> I need to get rid of some of this junk lol