The barrel rib should be re-soldered so you don't get any splitting.
The overall length must exceed 660mm, and the barrels I would go to 18.5" (470mm) to be on the safe side. The only way you can go shorter is to buy one that was manufactured with that barrel length. Those gnarly Dlask Arms 870 pumps, or those junky Dominion Arms that I wouldn't touch with the proverbial 10ft (3.6m) pole.