Mobi to ePub conversion - cover aspect ratio being messed up

Yesterday, I uploaded a revised version of my Edgar Rice Burroughs "Barsoom Omnibus" to the library here at MR.

The Mobi version of the book, created with Mobipocket Publisher, has a cover which looks absolutely fine. The cover is 600 pixels wide x 800 pixels high, and is displayed correctly.

I then used Calibre to convert the Mobi version of the book to ePub, and in the ePub version the cover appears vertically compressed.

JSWolf has kindly looked at the ePub for me, and advised me that the ePub cover incorrectly has:

whereas it shouldbe:

Is there any way to change this in Calibre? The cover was added to the Mobi book in the usual way (ie via the "Guide" section of the OPF), so there's nothing explicitly in the book which determines its aspect ratio - I simply want it to use the full size of the supplied cover image - 600x800 pixels.

If it would be useful to see the books, the Mobi version is at:

http://www.mobileread.com/forums/showthread.php?t=12782

and the converted ePub is at:

http://www.mobileread.com/forums/showthread.php?t=54599

The conversion was done with Calibre 0.8.46 using default conversion settings.

Any help with this would be much appreciated.

Thanks,