TNG TM says that under most operating circumstances the M:AM ratio in a GCS is 25:1 and that it gets up to 1:1 as it approaches higher warp factors.
What if the thing that makes post-Excelsior designs special is that they're able to go with a pure 1:1 M/AM ratio rather than having to have reactant deuterium to be used in a way that can be energized. That is to say, things like the swirlie chambers probably are some sort of anti-matter spiked fusion plant rather than a pure m:am 1:1 reaction. Hence you'd need the stuff to be reacting at the whole thing of the chamber, rather than in a single reaction chamber.
Also, a XCV-330 with a 5km diameter engine would at least be the overall size of Earth Spacedock. Basically we're talking about a Babylon station sized thing.