Do you understand how a O/U's barrels are made… typically, the barrels are secured at the block and at the muzzle, the remainder is unanchored. Some of my O/U shotguns have side ribs, others are open. Typically, the side ribs are decorative and add little structural support.
When you cut the barrels behind the muzzle, you are removing their structural constraint and all regulation may be lost. Before you worry about screw-in chocks, you need to work out how you are going to structurally reconnect the cut barrels. If you simply ignore re-anchoring the muzzles, what do you think is going to happen with those barrels the first time you light off some of those Dixie Darlings? You might consider using something like the Krieghoff barrel hanger.
Image: Krieghoff International
On the other hand, some O/Us have straps joining the barrels as part of the forearm's mounting design. With the barrels cut short enough, these straps may provide adequate support.