How does that sound? It still sounds DANGEROUS. Yes I know my opinion is not the popular one and I am not giving you the answer you want to hear. Let me tell you why I am so adamant about this subject.......
Years ago, one of my best friends, a professional welder was given a tank to cut open to make a bbq grill. The 55 gallon barrel had been used to store an industrial solvent. The man who brought in the tank had assured Gregg that the barrel had been cleansed and purged and was ready to be cut. Gregg took the man at his word because the man had brought tanks in before for the same process. The tank blew up on Gregg taking half off his face and one of his eyes. It didn't kill him right away, but several years later he died from complications from plastic surgery done to help correct the holes that were still inside his mouth. After the accident, Gregg had at least twenty sessions of corrective plastic surgery on various areas of his face. This is why I try to warn inexperienced people not to try this at home.
I am not trying to persuade you from using the tank for your cooker. I have made my own cooker from a 275 gallon fuel oil tank. I am an experienced welder and I know what I am doing, but I had someone else cut that tank open for me.
One of the problems with using a used tank for a cooker is the tank's history. Do you know for a fact that the tank was used for was storing diesel? Just because it smells like diesel on the inside does not assure that that is all that is inside it. There could be other volatile substances in there as well.
I still do not understand what the dish soap is supposed to do as to removing ALL the residue inside the tank. I see that advice given all the time, but I see a lot of old wives tales given on how to "safely" cut open used tanks. One question I am going to ask you is where do you plan on depositing all this water? Five hundred gallons is a LOT of water!! By all rights, once the water enters that tank and mixes with whatever enters the tank, it becomes a hazardous substance. Are you going to let it drain in your yard and pollute the ground water or do you plan on letting it go into the sewer system? These gases could collect in the drainage pipes or sewer and ignite or explode. Either way you are not doing your neighbors any favors and you could be breaking the law.
Using a grinder could be considered safer than using a torch as the only gases introduced inside the tank are from the atmosphere. But the chance for explosion, although reduced, could still exist. Diesel has a flash point between 126º and 205°F and that flash point could be lower if other flammable residues exist.
You just need to ask yourself if taking this chance is really worth it. These people who are trying to "help" you by telling you methods that "work" are doing you no favors. Although that method may have worked once or twice before, it may not work 100% of the time.
I am sorry if I am peeing on your bbq fire. As I stated earlier, the money spent to have a professional do this part of the building process is well spent. It will probably be cheaper than a hospital bill that could occur too.
Good luck with your project.