Would the proposed Long Beach-sized Aegis cruiser be large enough for an autoloading 8-inch gun?
Short answer
Yes, but restoring the gun would also require rebuilding the industrial and support infrastructure that once produced it.
Notes
The larger counterfactual cruiser has enough displacement and deck space to accept an autoloading 8-inch weapon. Such a gun would add substantial surface and shore firepower beyond the standard 5-inch battery. Physical room on the ship is only part of the problem. The United States would also need to recreate the industrial infrastructure required to build a weapon that is no longer in production. Shipboard feasibility is therefore separate from the cost of restoring a dormant manufacturing capability.