I agree. That is a great verfication tool. Here’s another link for a mustang door/data plate decoder:http://www.mustangdecoder.com/decoder.html
There used to be one that you could plug in the info, and it would instantly give you the info. I did it on my '68 Cougar. That price seems awful low for a K code car, but stranger things have happened. I lucked up on both my '68 COugar and my '65 Galaxie.The Galaxie was the real bargain. 45,000 actual miles original drivetrain, interior, reclinging passeneger bucket, console, etc. and I got her for $6000. Keep in mind that a '65 Galaxie 500xl with a small block isn’t going to command as high of a price as a K-code mustang.
So you may be on to a deal. I hope so. Let us know who it goes. The HIPO 289 had screw in sstuds and guideplates (but that can be done on the non hipo heads so look for a 19,20, or 21 emboosed on the leading edge of the head casting). The harmonic balancer was thicker on the hipo unit (again something that could have been changed out).
Those are a couple of quickie references,but unfortunately are also things that can be changed out to make a non KCode 289 appear to be a true K-Code engine.
Check out the link that Kyle sent you. And hopefully someone who has a K code car that they can reference for you will chime in.