Hey, the Montana is level 103 and it's guns are level 100, with a second set available later at 110, so you're able to use the ship.
The guns you want to be using are the Lvl 100 3x 16"/50 cal Mark 7 Mod 1. Every BB5 also has Lvl 110 guns which generally do not work very well, USN is an exception, but the 110 guns for the Monty are the Lvl 110 2x 18"/48 cal Mark 1 guns, which are less popular mostly for being duals, but you can't use those atm anyway.