Daryl - these are fitted directly to the frame, no subframe francotte-style with all the parts mounted on it. Changing the extractor means taking down the action (well, at the very least popping the breechblock).
Hammersmith, the GP is wider and has some subtle changes in geometry which makes a 12 gauge a better fit - according to FDH, the round is a tight squeeze in the standard military action.
Even better would be using one og the Greener EG or similar actions, they are not takedown, so stronger.
FWIW, I have a Greener GP takedown in 45-70 - it still has only 1 (one) barrel, which is locktited in the receiver. I prefer an excuse for another rifle...