I think the distortion in this one is due to barrel distortion in the lens. Does overlapping adjoining images by 50% reduce this?
I think it's important to keep the camera as level as possible
when making panoramic shots. By overlapping the image, more
points are available to line the image up and minimize any
distortion introduced by the lens. I've had good luck with
images overlapped by 25-30 percent.
Couple of other points. Using a wide angle lens can result in
distortion at the edges. If you look to the left side, you'll see
tall buildings leaning. Also, make sure the exposure is consistent
between images in the set.
Look towards the left side of the bridge and you can clearly see
the seam created between the two images (this is three images)
and the difference in exposure.