Hi Andrew,
This stuff gets tricky - printing panos. Let me take a stab at it based on my experience/recollection printing panos, which I haven't done for a while.
Firstly, the "width" is the short-dimension, not the long dimension. In printer-driverese it relates to the width of the printer carriage. The "height" is the long dimension, so you need to start thinking of your pano as a Chinese Scroll that hangs in portrait mode on the wall and relate your print dimension settings to that scenario. So the "width" is going to be the left and right sides and you want those margins to be equal if you want your "scroll" centered down the page, and you should reverse the width and height settings for the cell size, versus how you have it set up in your screen grab. This philosophy of dimensioning needs to be carried into the Epson driver for guaranteed success. Go into print settings and create a custom page size set-up (as well as all the other driver settings you use) that corresponds with the needs of your pano as discussed above, and save it as a custom preset with its own distinct name. Then, any time you need to print this pano, including the first time, you would select that preset. See if all this helps.