When you design your ships, you see a "Weap Mount" button on the right. That allows you select the stronger size for available direct-fire weapons. Do notice the minimum size(tonnage) of the ships required for those mounts.
In the regular SEIV game, you'll never be able to Large/Heavy/Massive mount weapons on destroyers, no matter how much space you leave available. In addition, the increase in range applies only to Starbases of any size, not to ships or weapon platforms....again, in the basic game. I know nothing about most of the mods.
Watch out for those construction and maintenance costs as you raise your warships' firepower!!