Looks to me like you don’t have any features that hugin can use to account for the fact that you tilted the camera up when the top row was shot. Probably tilted it down as well when the lower row was shot. If you look at the perspective correction tutorials on their site you will see what type of features it needs. They’ve changed it but it should still cover what settings are needed and what it needs to do this
http://hugin.sourceforge.net/tutorials/architectural/en.shtml
Maybe each shot could be done manually and then formed into a panorama but I suspect you would need a lot of overlap.
There is next to no distortion on the lens you used if it was on a crop camera and none that would be bothersome on a shot like that if it was on full frame.
John