[RJV]

You shouldn’t expect there to be a direct 1 to 1 correlation between engine capacity and horsepower because so many other variables will differ. In your example there is a 12 % increase in torque. With different years or models they may have different levels of emission controls. And you can have a 3.7 litre engine with cylinders of a particular bore and stroke and another engine also of 3.7 litre with a different bore and strike and, all other things equal, they will produce both a different power and torque level. Then the injection systems can differ. And then the power curve could also differ ( the hp rating you refer to is just a single point on a graph of hp vs RPM) I could go on and on ......

 

[RJV]

55, 500 Are fine, but they very few in my country, especially in decent condition.
Is the low RPM power difference noticeable between 320 and 350?

Why don’t you do a google search for the power and torque curves for both engines. A picture (graph) is worth a thousand words - the answer will be obvious to you and you will save others from typing a thousand words.