You'll never find a 56" alcove tub...tubs are measured from the stud to stud without any finish on the wall, and you will find lots of 60" tubs. If after removing the walls, your space is really only 56", you might find a 54" tub (they tend to be made in 6" increments from the 'standard' 60" version). There are lots of choices, some have a fixed apron, some have a removable one. There are many available with a tiling flange for an alcove installation. CI will last the longest and look the best (finish wise, maybe not style). There are lots more choices in acrylic, and it can last, but needs more careful installation to make it feel nice and sturdy.