Hmm this should be fun! If not I may have a 300R case to sell because the cooler is going nowhere
Can I mount the cooler so the fans face up or must they be vertical?
I'm fairly certain you can mount it both ways:
The SoliSku universal mounting kit provides an easy-to-install rotational backplate for Intel LGA 2011/1155/1156/11366/775.
(Source: Phanteks site linked earlier)
Note that this may actually make the airflow worse and it might not even fit your case that way.