Nice! How are you going to regulate? Looks like the ribs are already sweated in. Or is the left barrel floated at the last 1/3, like Merkel DRs with the adjustable setscrew regulation at the muzzle? I did a 45-100 DR on a Merkel 12 ga action, and figured out that they use 2 setscrews at 45 deg angles in the front sight block to move the left muzzle for fine tuning regulation.
The ribs are sweated. If the barrels need to be moved for regulation they are placed in a fixture and the forward 1/3-1/2 of the barrels are heated until the solder releases, then the barrels can be moved either together or independently via screws and allowed to cool.