Are we talking about the 55Hp who was introduced in 1968 or the 55Hp from 1977 with twin carbs (818cc volume) which actually was the 60Hp from 1974 (with Magna Power II ignition system)?
I suppose we are talking about the early 55Hp from 1968.
These engines has the same LU as the 35 and 45 Hp, same ratio same prop shaft and drive shaft. It will fit right on.
Through the years they change the coupling of the shift shaft and the roller bearing for the pinion gear too.
The 50Hp Force engines has the same LU.
My experience of these LUs is that they are pretty decent and very simple.
There are no shiming for the pinion, forward and reverse gear.
The pinion gear is just floating on the forward and reverse gear.
The most important thing with these LU is to adjust the shift rod properly and sometimes it can be a PIA to adjust it with the remote control if there is much play in the cables.
Otherwise in my opinion these 55Hp Chryslers and the 50Hp Forces are good engines.
Have in mind if you do not maintain OBs it does not matter which brand you have.
They will fail.