When selecting a head, one wants to pair the legs with a head that will not compromise the stiffness of the system as a whole. On the other hand, a head that is too big adds diminishing performance increases and just adds weight. The LN-324C is a decently stiff tripod and so wants at least a mid-size ball head. The tripod weighs enough though that there is very little downside to just using a full size head, which typically has better ergonomics.
Below is the optimization result for selecting a ball head. This chart plots the stiffness to weight ratio for this tripod and each ball head that I have tested in blue dots versus the stiffness of those heads. The red line is a generalized model for a strong performing ball head. The higher up on the this graph the higher stiffness/weight performance of the system. More information on this analysis can be found in the ball head choice optimization page.
This chart is telling us that you want to pick a ball head that has a stiffness of at least 3000 Nm/rad. Heads around this range are typically smaller than is appropriate for these legs though. Not that there is anything wrong from a performance perspective, but it looks pretty silly to have too small a head and you may as well get the ergonomic benefits from a larger head. As we can see there is very little overall performance drop from getting a full size head in the 6000-8000 Nm/rad range.
Note that these recommendations are limited to the tripod heads I have tested. The more general discussion above should provide adequate guidance if your head of choice is not listed.
Sirui K-40X The price to performance of the Sirui K-series heads is unmatched, and should thus pair well from a stiffness and value perspective. If you are looking at buying this tripod, I am assuming that you are a cost conscious buyer, and so aren’t looking for a head that costs just as much as the legs.
Sirui K-30X The K-30X is just a smaller version of the K-40X. Purely from a performance perspective, this is the best head for the LN-324C. It is plenty stiff and is significantly lighter than the K-40X.
On the head ranking page, you can see that there are quite a few mid and full size heads with the requisite stiffness to perform well with this tripod. Most though are over $300. If you are considering one of those heads, I would instead recommend diverting funds to get a better set of legs than the LN-324C.