Don't try to do anything as a unit, yet.
First establish where the face of the breech plug is, on the outside of the barrel's side flat. You may have to move the barrel back in its channel so that the pan lines up with where the vent is to go.
Remove the breech plug, underlugs and sights. Inlet the barrel alone, according to the first step.
Now reinstall the plug, having filed a pleasing finial on it's rear end, and bent it cold to the contour of the wrist. Inlet the breech plug and its tang.
Reinstall the lugs, and mark their position in the channel by measuring or with transfer colour like inletting black or prussian blue pigment. Inlet the lugs into the channel.
Mark the lug's position on both side flats of the barrek, and transfer those marks to the wood. Measure the thickness of the barrel at each lug, and transfer that dimension to the wood. This will locate exactly where the pins or keys will have to go.