If you get dinner finished by 6, you 'might' be okay, but it's going to be close. If the monorail is just leaving the platform when you arrive, you may have to wait about 10+ mins for the next one to come along. You could use the boat..it goes directly to the MK from the Polynesian, but you won't know if it's just left or is just arriving until you get there.
Once on the monorail it will take you about 10 mins, perhaps 15, to get to the MK. Then you have to go through bag check, the turnstiles, then walk to your final destination..that alone will take about 10 mins.
As you can see, it all has to line up perfectly for you to make it there by 6:30. You can try arriving at 'Ohana early but if you arrive too early,they will tell you to come back...they seem to have adopted the policy of arriving no more than 15 mins before your ADR time.