water heaters have a fiberglass liner in them and that is a PAIN to get out of them. sometimes the bottom of the heater has about 6" or so of lime buildup and you need to account for that. i cut open one recently and had to chain one end to a pole and hook a truck to the other end and jerk it apart with the truck. there are usually multiple heat exchanger tubes on commercial heaters so you have to cut them also and remove them..... in other words your prolly better off to scrap the waterheater tank and try to get something else. that's what i did.