Awesome! What did you end up doing with conversion size? Did you go for fixed dimensions or Change width and compute height to keep proportional?
EDIT: Based on the code at the link it looks like he sets the height to 330 but does not change the width so my guess is that it maintains aspect ratio and the heights will all be the same with some variation in the width. Great solution!
EDIT2: Confirmed that aspect ratio is preserved. I prefer all my covers to be the same so I added the following line to the end of the python script in the zip file in order to ignore aspect ratio and make the cover 216 width and 330 height.