What is the best way to position a cover in a mockup?

Start with Fit Vertically for most portrait book covers, then pan the image horizontally if the mockup area is wider than the cover. Use Fit Horizontally when it is more important to show the complete width and you can accept some space or cropping above and below.

Check that the title and author name remain readable and that faces or focal objects are not cut at uncomfortable points. A small amount of natural cropping often looks better than shrinking the cover until it leaves obvious empty space.