My 1/350 USS Montana project.

As these ships were never built, any light AA configuration is purely conjectural.

That said, if I were doing this I would ask the question, "What AA armament would have been likely had these ships been built?"

My guess is that there would have been no 20mm guns at all. By the end of the war, 20mm did not have sufficient stopping power. The Iowa's had them removed soon thereafter. With that hindsight, methinks the 20mms would have been sacrified for all the 40mms possible.

The AAW systems for the Montana class never reached final design due to the class being canceled.

My Conjecture.
1. The Montanas would have fitted the new model 5inch/54 semi-automatic guns. These guns were used in the Midway class CVB.
2. Twin 3inch guns with VT shells would have been fitted in place of half of the quad 40mm. Twin 40mm would be fitted in place of many 20mm.
3. 20mm would be fitted but, they would be in twin mounts where 40mm could not be fitted.
4. Increasing the number of 3 inch is preferable to increasing 40mm because 3 inch shells can be fitted with VT fuses (proximity fuse). VT greatly increases the hit probability even with a reduced fire rate.

Regardless of the weapon fit which you choose to go with on your conjectural build, the basic ship's ground tackle handling requirements would have been met first.   Anchor chain bolster plates, capstans, chain locker access and similar gear would have been installed as part of the basic ship construction.  

Tertiary AAW armament (20mm, 40mm, and/or 3-inch, along with their applicable directors) would have been installed on a space available basis, based on the current threat expected.

As the Montanas would have been a more advanced ship design, it could be that they would also have survived to see duty in modern times as the Iowas did. Phalanx CIWS, Harpoon, RAM and Tomahawk launchers would be installed in later modifications, not to mention new electronics and their antennae. That would be a good project for some enterprising soul.