Measuring for a new tub

My house is about 15 years old and I want to remove the original one piece tub/wall surround system and replace it with a separate tub and ceramic tile on the walls. When I did the measurements to see what length of tub is needed, the wall measures 58" across, but a standard tub measures 60". My question is this. Should I be measuring from stud to stud for the new bathtub or from wall to wall? My guy who is doing the tub installation says I need to get a special order 54" tub, but I'm having a hard time believing that I can't install a standard size tub in this bathroom. The current tub/surround setup fits between the 2 walls and there is no room for error or expansion. Anyone have any advise on this? This is my first bathroom remodel where I've decided to replace the tub, so I have zero experience with this.

Are the existing tub walls glued to the drywall or are they attatched to the studs?

tub

Stud to stud, and if the installer doesn't know that, then you really need a different installer.

Cass,
I haven't removed the old unit because of this tub "issue" so I have no idea right now what it is attached to. I suspect it is attached to the drywall, as the surround piece sticks out from the wall at the point where the surround begins about a 1/2" - 3/4".
hj, thanks, I had my suspicions about that.....lol
This is an awesome forum and you have been very helpful!
Cheers!