Import a 513x513 Texture2D (heightmap)

For my new game i need a terrain that consists out of 256x256 tiles. Because of the 65k limitation i have to break the terrain up in several meshes of 64x64 tiles. This means that for making a mesh, i have to have a 65x65 pixel heightmap. Impossible! because the Texture2D needs to be the power of two.

If i stick with chunks of 63x63 tiles (64x64 pixels from heightmap) i end up with a big problem: After setting the position of the chunks next to each other i end up with 252x252 tiles because the meshes are uneven.

63+63+63+63 = 252 tiles instead of 256.

Is there some kind of solution for importing a power-of-two plus one texture?

(i didn't use the Unity Terrain because as far i know it is not suitable for a 2.5d city building game.)