A trick of old European barrel fitters is the following:
Remove the cross pin. Fit the barrels in the action so they fit every where.
Once you have that fit done, place a 1mm thick shim at the bottom of the standing breech on the flats, put the barrels on, clamp them tight down on the flats and against the breech face. In this position they will make full contact at the front, on top of the cross pin hole, and the back, the bottom of the breech will touch the breech face. The shim does not allow the barels to sit down on the flats.
Now mark through the cross pin hole the circle of the cross pinn on the front lump.
Cut, fit the front lump and the barrels are ready to smoke fit down onto the breech.
As in all cases of seemingly complex procedures, there are simple ways to do it.
Additionally you can slot the front lump for a removable shoe and that way you have the luxury of making many fitting mistakes since the most you can damage is the replaceable shoe and not the lump itself. You can do the same on the lockup area of the rear lump.
I did this removable shoe on my SXS prototype, made with 3D printing but barrel fitted in the traditional manner. It was shown here some time ago.
