The tub needs to go in level, and then the walls will fit.
Use a level to set the back supports and shim the end of the tub that has too much space to the floor.
Find the high point you have to work with, mark that on the wall and then use the level for the rest.
With a floor out of level like that, it may be helpful to use something like mortar in spots under the tub. I look at that as small piles that have room to squish as the tub is dropped into place.