I would say get risers for all the cards as there is going to be some major heat on the middle card(s) being so close.

Do these cards need powered risers?    I'm about to add a card to my rig and only got regular 16x risers(installing this weekend).
I thought that the direct PSU -> GPU power connect allowed a normal riser.

btw i'm no expert - have only built 1 ltc rig recently and still learning.