On a 65, a factory GT only came on four barrel A and K code cars. NOT the C code 2 barrel carby cars.
Check the VIN number.....it won't be a K because they are worth a mint, so you'll be looking for the VIN's 5th digit to be an A. Then it's got a chance of being genuine.
This is shown on a long thin tag on the inside of the left hand door near the lock.
What's hard is that any 4 barrel car could have been swapped to a 2 barrel on a genuine Ford inlet manifold......a lot can change over 44 years.
Look for round dial instruments like the 66 model, as GT's were fitted with these. If it's got a standard 65 sweep speedo beware. However, deluxe pony trim cars were also fitted with the dial instruments so that feature is not unique to GT cars.
Perhaps use the 2 barrel carby to request a price reduction, grab it then go out and get ya 4 barrel.