Those three cars listed above ran 13s in completely stock condition. And, sometimes, even when hampered with a balky manual transmission (many car mags preferred Mopar's 3-speed automatic Torqueflite to the Mopar 4-speed manual transmission). A car running in the 13s is fast then and now. And can actually be dangerous in the hands of an inexperienced driver.
The GTX you are referring to was a convertible... a several hundred pound handicap right there. A hardtop or coupe would have been a little faster. In any case, that GTX ran a best 1/4 mile of 13.97 @ 103.50 with only the driver aboard, 3.23 gears and factory '60s tires. Very good for an unmodified street car with an engine detuned for street use.