They should be fine.  Make sure you have good airflow in the proximity of the intakes and they'll stay cool.  Even the boxy cards are pretty good at sucking air.  You probably want to run the GPU fans at 80% or so.

If you're an open air setup, PCI-E slots have a good bit of "flex" built into their spec.  You can put a 5mm spacer between the cards to increase airflow to their fans without affecting the electrical connection and open up space for them to breathe.

I would suggest against using risers unless required, as they can cause reliability issues.  The're a possible solution, but I strongly discourage their use.  Try without them first.

Source: Extensive experience building dense, multi-GPU rigs for password cracking, including fairly reliable "impossible" systems such as 4x 6990s in a board without extender cables.