When "El Lobo" is chasing the Mustang, damage can be seen behind the driver's side front wheel as the car comes around the corner. The damage then goes away until the pickup rams the car for the first time a short time later.
Lopez cut the gas tank of the GTO apart to retrieve the money. A short time later he drove it to the used car lot. Since the gas tank for a 1966 GTO would be very difficult to come by, the car would be inoperable until the gas tank was replaced.
Annie describes the GTO as a 1969 model. The car shown had vertical headlights, making it an older (pre 1968) model, likely a 1966. The 1969 GTO had horizontal headlights.