The best choice might be HFSS.
Some FDTD softwares also have periodic boundary conditions, but I am not sure about the accuracy of the algorithm. Can anyone comment on this? One is called the split field formulation?
IE3D 9 also has option for setting periodic boundary, but it is not clear to me how they have implemented it. They give an option for setting the number of images, so it appears that the green's function used is not for periodic structure. It takes a lot of time to run if you set the number of image too high (like 20). So the accuracy might not be good enough.
Another MoM code, EmPicasso (?) can also simulate periodic structure, but the version I tested (2 years ago) can't have metal touching the periodic boundary. So it is not useful for your case.
Although I think HFSS is the best choice, I have never seen an accuracy test on it. You might also check out the FEMLAB, another code that uses finite element method.