I want to replace a 60 cast iron tub with a 60 fiberglass shower base and enclosure. The bathroom is on a concrete slab. I have not been able to find a 60 unit that has a left hand drain that will mate with the existing tub drain. What are my options? Is it possible to extend the drain pipe in order to install a unit with a center drain? Remember this is on a slab. Is the old drum trap a concern? Any help would be appreciated.