Might wanna seriously consider changing out the tub's mixer plumbing valve and pipes during this project if its old. Perhaps you could replace it from the backside of the wall so as not to disturb the mud wall you're attempting to save. At no other time will it be as simple to replace. And if your current mixer valve has two handles, consider how a replacement will most likely have a single handle in the middle of the wall.
...much easier to replace now and be done with it for years to come.
Tonto Goldstein….. but my friends call me Bubba
"The best way to get a bad law repealed is to enforce it strictly"
- Guy on the five