This should show you how it's done it's a curved wall with a capping that you control the texture mapping of all sides of the arch, well in this case the brick capping, the principle is the same.
Looking at you arch I think you are using texture by object and not by components, I am not at computer but you choose by component not by object and this will give you the option to change all sides of the object.
To adjust the bricks you may need to rotate as well as scale the texture.
There a few other videos on my site that may assist also, look at the table one.
HTH