there is no way in the world a a stock 289 is going to get into the 13. With my 302 and f4b intake holley 600, and t5 trany, my best run was a 15.6 on street tires. With my built 289. I am only running a 13.4. Sure that is on bald street tiers, but still i had a trac lok and 4.11 gears.
I work for a book publisher that distributes all the old road test books, and rarely did a 390GT break into the 14s. The 66 289 k motored cars ran about 15.2 and most the 390GTs ran 15.2 to 14.9.
Have you ever run the old polyglass (fancy word for plastic) tires? That 390GT you mention came with the then top-of-the-line F70-14 tires that translate basically to 205s. Put some 245 series dot legal drag radials on that 390GT and you've got something. Plus the stock 390GT exhasut was a joke, and it had terrible cast iron exhaust manifolds.
Now, I also refuse to believe that a stock calssic stang with a C or A code could hit the 13s but that was due to the cam more than anything. Those things were WIMPY. Even the venerable K code cam was a slouch compared to modern standards. BUT, give them a decent cam/stall/intake/carb/tires/exhaust and they can even with basically stock heads.
That was my point Edbert. Hard to imagine anything with 350cfm of flow and 2.80 gears could crack the 15s, much less 14.6.
While the cars had bad tires in those days, a stock 289 C4 with 2.80 gears wouldn't need any better tires anyway. I had a 66 Comet Cyclone GT with a 390 and sometimes it would hook up, and sometimes it wouldn't. I was running highway gears as well.