Why not just Custom (Clean) install Win 8?
During the download of Win 8 choose Save Nothing, then choose Custom Install. You will be given and option of Install now, Install by Creating Media or Install later from desktop. Choose Install by creating media. This will allow you to burn the ISO file to DVD to use. (Right click the ISO file and choose Burn Disk Image). This creates a bootable DVD. This will also then give you an installation media for future use.
Now boot to the DVD and install Win 8 from there without having to do anything to Win 7.