When done you have to choose how to handle the side galleries. Either you remove them from the ship or from the new stern gallery. I prefer to keep the ones on the ship most often as they are visually best aligned with the gun decks. The scale may differ a little and therefore faults in height is easier to hide by filing the new stern gallery flat. It also takes care of any differences in width.
After having made sure the pieces fit together well spray the new stern gallery with a base coat to make it easier to paint. When dry I used superglue to attach the new stern gallery and then paint it in colours blending in with the original. When the old stern gallery is removed you sometimes have to reattach the rudder. I've sometimes had to cut it down a little and glue it back on.
There are several ships that may benefit from this. I have done it to HMS Victory to add the stern gallery of HMS Royal Sovereign to make a more impressive version of Admiral Collingwood's flagship.
Or if you want to add a smaller stern gallery they may also fit quite well. This is my shortened version of Courageuse as HMS Surprise.