Probably best to follow GP's advice: simply use the 8th ed rules and see how it goes.
You could take a monstrous cavalry unit that exists in the 6th ed (Dragon Ogres?), see how that unit is depicted in the 8th ed, and try to backtrack the 8th ed Demigryph logic back into the 6th ed logic, and adjust their point value accordingly. Then again, I remember hearing that 6th-7th ed monstrous units (cavalry on 50mm squares and infantry on 40mm squares) was underperforming. They definitely lacked many of the 8th ed special rules. So you would likely see the Demigryphs' power diminished. Then it's about finding a points value that feels right.
-Z