60" is from stud to stud. The finished length of tub alcoves is frequently about 58 whether it's cement backer and tile or a fitted fiberglass surround. You should be okay

Thanks, pal. This was originally built as a kit house, so I'd be surprised if it weren't the simplest and most common size; I was just surprised because 2" seems like a pretty significant difference!

The 58" is from sheetrock-finished wall to sheetrock-finished wall, measured outside of the tub, in front of the fiberglass surround. The fiberglass parts appear to be very thin, maybe half an inch (I didn't take the fiberglass into account in my measurements). So, would your explanation hold even in this scenario?

If there is sheetrock behind the surround (sometimes there is, sometimes there isn't) that accounts for 1-1/2" inches (3/4" drywall on each side). At any rate, 60" is a nominal measurement --a 60" tub is made for a 60" space and the real measurement is slightly under, by varying degrees.

Thanks for clearing that up!