Making anachronistic ship designs

My approach to making an anachronistic design of a ship dating from the 1860's, usually dated after 1905 but before 1914, is to take the original dimensions and make drawing to look like what you want. You can see the sort of look that has struck my fancy in the past. If the original ship had a waterline length equal to the length "between perpendiculars", I might multiply that by 1.045, or some number like that, to produce a waterline length for a more modern looking hull. The trickiest part is choosing an armament. For ships such as screw frigates, corvettes, or sloops, with many guns, I would rather use a smaller number of larger guns. I am not sure what system I used 18 or 19 years ago. I am not sure of a principled way to do it. At this distance, I would guess that I chose the armament to be rather heavy, using standard sized guns, and used that. For wooden ships, I would rather not use much more than a protective deck and possibly a torpedo bulkhead. Obviously, you also need to armour turrets and barbettes.