I could be all wet but I thought HiPo 289 cars had 9 inch rears and larger drive shafts? BTW fleet of shock 289 HiPo powered 64 Comet Caliente's held the closed course speed record for the fastest 100K miles for 20+ years. 105+ miles per hour was the average speed over the compete distance. The cars were refueled and necessary fluids added on the track at close to full speed using a chase car when possible. The drivers were skilled mechanics and the cars had a 200 lb tool box bolted to the floor filled with tools and parts. This was done so in case of a break down the cars could be fixed on the track. In the end it took 42 days to rack up the 100K miles. In 1965 Mercury took several 65 models and set an on road speed record driving from the southern tip of South America to Fairbanks Alaska non stop. If I remember correctly it took 40 days to complete the trip. The average speed has slipped my mind. Yup 289's are some tough SOB's.
Al Bundy is correct. For some reason the 64 Comet 289 4V has the VIN code "K". It is not the HiPo 289. A few, oh hell it was many years back this was discussed over on the Yahoo Cougar group. But just the same, Sweet looking Comet. Oh if you read the text, it was originally a 3 speed car too.
No matter what it is, cars included, you should do your homework and know what you're buying before you spend your money. This guy represents the car as exactly what it is. If someone paid too much for it thinking it was a hipo I wouldn't feel sorry for them.