HowTo: Gluing stainless stell together?

Hello, I am planning to replace my existing cast iron tub with a new "step-in" type tub. I believe that I understand how to remove the old tub drain and fixtures and then break the tub up with a sledge for removal. The tub is in a wall alcove with tile from the edge of the tub up to head high on the back and both sides. Since the sides of the step-in tub are twice as high, or higher, as the old tub, I am hoping that I will not have to replace the tile. That is, I am hoping that I can break up the old tub and remove it without having to remove more than a few courses of the tile immediately above the old tub. The old tile looks good, and if it were not removed, my hope is that the new tub could be installed and sealed against it. The step-in tub is slightly narrower and 6 inches shorter than the old tub, so it would fit, but I would like to keep retiling to a minimum, if possible. I wonder if it would be advisable, if this is attempted, to try to free the old tub from the wall (or the first course of tile connected to the tub) before breaking it up. I recognize that this may be a dream, but I would appreciate any help. Thanks.

I need to replace a corner shower in a basement. Since we want a tub, but there is really no room, if colour is not an issue, and a proper frame was built, could a fish pond liner (since they come in so many shapes and sizes) be used as a custom bathtub? If it is possible, what would need to be done differently than putting in a fiberglass bathtub? Thanks bunches. Leslie