If the Montebello is referenced as a 118guns or a 120, i guess it's because the largest Sané 3 decks were tagged as 118 guns until 1824, when the Commission de Paris whose aim was to reforge the french navy, reorganised the ship ranks : 120 guns, 100 guns, 90 guns (and 80 guns). New 120 guns were planned, whereas the old 118 guns became 120 guns.
The 118 guns were fitted at times with more than 120 guns, up to 132 (130 or 132 for the Montebello, i'm not sure)
When Montebello was refited in 1822, it was slightly enlarged, the forecastle and the quarterdeck were joined together so that a kind of 4th deck was created.
if i'm not wrong the Montebello and the 118 guns were sligthly longer and had a slightly more important displacement than the Santisima Trinidad.
Anyway, we don't know if the Montebello will be in game one day, but the Océan, for sure.
It will be a nice opponent to the ST.