That tub will be 60" long between the studs.
Just pick up a 60 x 34 shower pan, and then decide how to cover the walls.
Tile, Laminate, Cultured Marble
Or you can pick up a kit with pan and three walls. The disadvantage of the kits, is that they don't go very high, and you wind up having to drywall over the nail flanges.
A standard shower door is too high for the too short walls.
You spend less on the kit, wow! a big savings, and then you take the savings and give it to a drywall contractor.
Or have a cultured marble shower or tile, then you can go to the ceiling if needed without doing drywall repair, the doors will fit, and it will look better.
Since you are cutting up the existing tub to remove it, don't worry about the epoxy, that will be cut out.
Don't forget to replace the shower valve at this time.
Moen, Delta, Grohe, no Pegasus please.
The idea is to have a new pressure balanced valve that you will be able to get parts for in the future.
You can't get parts for Pegasus.