I wish I hadn't seen the picture of your tub before I went to bed last night. The thought of it interferred with better dreams. To gain a shower you need to have a side wall built with a valve, spout and shower head. The problem is the drain. Do you have access below this neat floor? Todays overflows come down next to the inside wall of the tub or in your case inside the walls of the tub and the drain shoe connects to the drain above the floor. I think you will have to replace the tub.