Your drain shoe is probably setting on the concrete. You need a bigger hole. This is not a hard connection to make. Because cast iron goes bad we usually replace the trap with a plastic one. Usually the center of the tub is not the same and we have to move it anyway.
Setting the tub correctly affects the whole job. Don't start the walls until the tub is right. Enameled steel tubs have a limited life due to chipping. It is not wise to use an economy product as the base for your job but that is your decision if you have taken limited project life into account.