How to convert Italian grades to American and U.K system?
Don't even try.

There is no "American grading system".  There is a fairly common 0.0-4.0 scale for "grade-point averages", but the meanings of those GPA differ significantly among different US universities, among different departments at the same university, and in some cases, even between different instructors of the same course.

The only reasonable way to judge what Italian grades say about your potential for graduate study is to compare them against other Italian grades.