As LXguy says, The "K" code or "HiPo"289 is pretty rare and not the same as a regular 289. Putting a 4 bbl on a 289 would not make it a muscle car engine.
I have had both 340s and a real "K" 289 Mustang. I just happen to like Mopars more than everything else but that Stang was an amazing car. In stock form the "K" 289 is a nasty little motor. It was stock with solid lifters, 4.10 gears and a 4 speed. That with the headers, it was really fast especailly 0-60 where most of the stoplight action was, still not as fast as a 340 set up the same way in the 1/4.
By the time the 340 was out the "K" motor was gone so its not like a buyers could have had a choice between the two. A better comparison to the 340 would have been the "M" code 351 C.
