As others have said, the 390 wasn't ever refered to as a HIPO. 390 GT yes, but not HIPO. The only HIPO was the K code solid lifter 289, which was discontinued after 1967 in the Mustang.
What the original poster may be thinking of is a fairly rare regional special called the "
Stallion Edition." This was a 1967 fastback offered through a Canadian dealer. They came with either the 289 HIPO or the 390 GT. The rear tail light panel was funky because it had Cougar tail lights! Very odd looking IMHO, but hey, it's rare!
More info:
1967 Stallion Edition