Trying to fit all contact surfaces simultaneously is bound to be hard. In your position I would use some tricks.

The areas that need full contact are the front lump and barrel hook on all sides, the front of the rear lump (the famous fitting on the circle), the breech ends to the breech face.

The fit must clear the smoke on the sides of the rear lump and between barrel and action flats to accomodate the flexing of the action on firing.

So, I would deliberately shave off a slice, let us say 1.5mm from the front of the rear lump, leaving a gap. This would allow attention to be focused on the proper fitting of the front lump and the breech ends. When these parts are fitted then I would solder a slice of steel on the front of the rear lump and fit that in so it makes contact with the action bridge. The barrels would then be fitted on the circle, and also would be easily refittable in the future if any play develops.

Breaking the fitting down into manageable chunks that you can do one at a time is preferable to trying to fit all surfaces in one go, this is my experience. Additionally the use of replaceable shoes where the do some good, as in the barrel hook, rear bite and the front of the rear lump, make sense from the repair angle. Check out the Parker collectors pages for the replaceable shoes on the bite, it is a nifty idea.

Hope the above makes sense.