Paint stripper
The "aircraft stripper" variety of paint stripper will most likely do the job but it'll be a messy, time consuming task. If you go that route make certain you have lots of chemical resistant gloves-- that stuff leaches through most gloves so I wear a couple of pairs of thin gloves inside of a pair of really thick ones.
You might be able to shorten the chemical stripping process by using a heat gun to strip the tar first. It goes without saying that you should neutralize the gas fumes inside the tank before attempting either method.
BTW, if your willing to spend a few bucks you could probably have the profesionally tank cleaned inside and out through either dipping or blasting. That'd be my choice because the DIY methods are so time consuming and nasty.
HTH,
///Mike