I am ready to install my cylinder heads onto the block and have a problem. I am missing two large water directors (they direct the coolant toward the valves) and one small water director (directs coolant toward the pre-cup). How critical are they? I've found some newer style large directors at Florin and haven't found any small ones.

It seems to me that if your coolant system is full and your water jackets are clean, the directors won't make much difference.

Any advice would be appreciated. 

The directors could be to minimize hot-spots by ensuring the coolant is flowing across all hot areas sufficiently.
5H-7071 is the part number for the director for the D330. It is still available from Cat. It uses the same size seal and ferule as the D4600 so they may be interchangeable. The D330 uses the same director for both purposes.

Like Phil says, they seem to be important in the way they direct the water flow which is quite important. I wouldn't put it together without them unless someone knows different. They are also in my 315 engines but I don't know what size.

Although maybe not as critical on the D4600 with twin heads I would still not put it back together without the deflectors. Same 6B6223 & 7B2418 deflectors used on the D4400 as well. Surely must be some used ones floating around out there somewhere. The 7B2418 seems to be harder to find...1.688" tall.

Thank you all for the advice. I suspected that would be the answer but I thought it would be worth a shot.