On the heatsink question, I do have one concern on the center of the K64 or wherever two or more K64s share an edge (e.g. "K256"). That's centralizing a lot of heat in comparison. Just wanted to bring the topic up so that whatever heatsink selected/recommended they allow for a clear airflow path when they are placed side-by-side.
I think plain finned ones will be good for this, as you could have a fan on two of the end ones and a metal plate on the next so that air flows along in "lanes", either that or an up/down like some people are using on the asicminer blades.