Barrels should touch at muzzle. And spacers should be used to keep tubes put in the middle. Usually there are 2 sets of spacers on factory barrels. Or you can just do one spacer in the middle. It really depends on the length of the barrels.
Regulation with shotgun tubes is a lot less involved than with a rifle. Just get them shooting good for point of aim on a patterning board.
Ribs can be tricky these days. You can make them. Or salvage from other barrels. You used to be abke to buy ribs from GPC, but i am. Ot sure about that anymore. I have a number of misc top and bottom ribs and coild help you out with something if you need.
Do your striking on the tubes before any assembly. Makes for easier work later on.
