Tub roughs are measured stud to stud. If you measuer 58 or 59 wall to wall , that is a 60 inch model. ( There is no such thing as a 59)
The life expectancy of acrylic or fiberglass is not as long as porcelain tub and ceramic tile walls. If you are scrupulous about wiping down the acrylic after ever use, never use any harsh cleaning products on it, etc., you can get many years. But it will not last 20 to 30 yrs+ like ceramic tile and porcelain.