I built my first big smoker out of a 325 gallon tank and had the same problem (door shape). I was told by one of the people as the gas company that the tanks are not round when made, but under pressure they round out. During construction of my firebox, I discovered the tank was an inch taller than it was wide.
So, to prove (or disprove this) take a measurement inside. Check from top to bottom and front to back. See if they are the same distance. If not, then you know the tank is not round.
If not round, use a car jack in the orientation that you want to increase (inside the tank) and expand the tank slightly and see if that helps make the shape of the tank more closely match the shape of the door. If it doesn't work on the first try, shift the orientation 15 degrees or so and try again. Do this until you get the best fit and weld in a brace that will hold the tank in the position you want.
I tried everything to change the shape of the door without success, but 1/4" steel doesn't give. Metals have a strength per square unit of area called Youngs Modulus (for tensional strength) and Shear Modulus for shear strength. In the case of mild steel, that number is 30,000,000 psi (Wikipedia. Sorry, It is the only reference I had). If a force is applied to a metal, it will return to its' original shape until the force is equal to or exceeds the Youngs/Shear Modulus. When it fails, the failure is along the weakest axis. It doesn't fail uniformly. If you have ever seen a rolled steel tube, they usually have a series of flat edges and are not perfectly round. So, I am trying to say that bending the doors uniformly at home without folding them up, is probably impossible. The doors will resist the bend until failure occurs and then the force being applied will continue the bend the door beyond what is desired. That is why I think you will have better luck trying to distort the shape of the tank instead of the doors.
I just accepted that the door would be leaky. So I put flanges on the doors and used high temp RTV as a gasket material to get most of it sealed off. The next year I built an upgraded smoker and just spent the money to have some 3/16" steel rolled for the cook chamber. The doors on that one are much better. Not perfect, but much better.
Good luck with whatever you do.
S.