How was the re-organisation of North East India complete and by when?

Presently north-eastern India consists of seven states- Manipur, Mizoram, Nagaland, Assam, Arunachal Pradesh, Tripura and Meghalaya. But this regional bifurcation occurred many years since achieving independence. Out of these states, Tripura, Manipur and Khasi Hill region of Meghalaya was the former Princely States. They ascended to India after the independence of the country. Out of these three states, Nagaland was created in 1963. Manipur, Tripura and Meghalaya became separate states in 1972. The last states to be formed was Mizoram and Arunachal Pradesh in 1987. But with the partition of the country, North-Eastern India has become a landlocked region. Still, the region is suffering many hardships and neglect in developmental terms.