I am an old 303 fan myself, since childhood.
I take it you will not be using the military wood, so will be able to easily free float the custom barrel.
For longer distance, I would suggest a 6.5 Epps- based wildcat, keeping things simple by using Ithe same or similar taper. Long barrel for better velocity. For closer in brush work, a big bore based on the original or Epps, and a short barrel. Because it is so smooth an action I personally would lean towards the brush gun. Of course a brush gun must be reliable for followup shots, hence stick to original taper.
But that action is pretty weak as noted above. Smooth as silk, super fast, but not stout. Did you ever see videos of the British sergeants doing rapid fire with that thing?