It looks like you are going to keep that head, and you should. I've heard good things about it. Good legs are worth it, but you know that.
400mm + the crop is hard to get steady. I did ok with my Carbon One (Bogen/Manfrotto) but with my Gitzo 1348 (found it for sale used) I can do 400mm really well and 400 +1.4tc on good days.