Right now, if you cut all of the singleton breaking (not counting lands) you would be 24 cards smaller. That would put you at 416.
Not saying that you should cut
all of the singletons, but remember that the smaller the cube the less important it is to have multiples to up density when you do the 2 player drafts. In addition, the smaller the cube, the less you need to break singleton to reach effect mass. For example, you could cut a swiftspear and noble, if you want to shrink your cube down.
As you noted, while you had a large cube, you also had a lot of chaff in it, which is ripe to rip out to bring your sections down to 50 per.
With your red section, for example, I would start by considering the cut of:
That brings you down to 47, though I would move that goggles to the artifact section, and honestly
reforge the soul would probably go, so more like 45, which gives you five slots to play with. That should be enough slots to toy with to run some more generic storm payoff cards, other red sweepers, a more flexible LD package (
pillage), or less restrictive to cast 2 drops (
abbot of keral keep looks like something this cube really wants, and a great place to break singleton).
If you really want to bring the cube down closer to 360, it should be possible, but like you said, tuff cuts.