And now I finally settled for a design for external boosters for F-58. Cannot attain orbit with this setup, but it does allow for atmospheric "skipping", which dramatically increases range.
The boosters are linear aerospike rocket motors with LOX and H2 tanks built into the booster pack. Each booster contains 10,000 lbs of fuel/oxidizer, which is enough for up to 100 seconds of burn time. Maximum thrust for each booster is 65,000 lbs. The boosters are generally retained for the duration of a mission, though can be jettisoned in an emergency.