Sorry if my words are going to offend you or anyone else who has contributed to this thread, but that is the way it goes.
What you are planning to do is very dangerous, ESPECIALLY if you have never done this before. Although diesel fuel is not as explosive as gasoline or propane, what you are doing is close to defusing a bomb! Now would you take first hand or second hand advice from the internet on how to safely defuse a bomb?
For every post that has been written in this thread on how to "safely" cut open a tank, I can tell you at least one way something can go wrong. Cutting open a tank with a torch or any device that introduces sparks inside the tank has killed and maimed more welders than any other welding process. This includes not only used tanks but unused tanks as well.
The only safe way to do this process takes two EXPERIENCED people. One to do the process and another with a monitor to make sure the inside environment has not reached an explosive level. As you cut the tank open, the gas levels change. Oxygen can displace the inert gas you have placed inside to prevent the tank from exploding. If your torch is not running properly, acetylene can accumulate inside the tank. The car exhaust idea can get you killed as well. If the car is not burning 100% of the gas, the exhaust itself can introduce explosive gases inside the tank.
My advice to you is to let a PROFESSIONAL do this!! They have the means and know how on how to do this safely. They can make a lot better cut than you can and it will make it easier for you to get the doors to seal properly. There will be plenty of cutting and welding to do after the tank has been cut open.
If you insist on doing this at home, at least do it out in the back yard away from the house. That way if tank does blow up, your widow will still have a nice garage.