Option One, Swirl Core:
This has primary deuterium tankage under the impulse engines, secondary tanks in the neck, and antimatter tanks in the humpback. While I suppose the vertical core could be ejected, it would leave a hell of a lot more inside the ship horizontally.
I'd go with this one, but add some kind of secondary reactor in the "hump" between nacelle pylons. Something almost NX-01ish.