I wouldn't recommend it in any case though! Just one plumbers torch (mapp gas) is a waste of time and money.To put it back is easier. A smaller torch (camping gas) is what you need. You should also be careful as heat can melt your previous soldering. It's all about destroying, oops, no I mean use one that you don't care to destroy it to get familiar with the process and then you can go on with the rest.
The screw driver is a bad company as Oyster49 said. I had cracked some tanks' ends. I still use it to rub the silver solder when it's liquid, or I hit the tank to remove some silver solder when the point is red hot. But those things won't work unless you see them happening or experimenting.
Another trick I do, I put fine sand in the tank. As much to cover with about 3cm the end I want to remove. Sand can retain high temps while brass spreads it away.
But that's something that you have to be sure that you will remove the end. Otherwise you will end up with a still dented tank plus with the sand inside! Not happy situation
Oh, I remembered something. My first attempt this way was with the small round tank of my FVP. This tank as it is small, with sand and only one mapp gas torch I manage to remove its end. But it needed a lot of time. I also create a lever with a screw driver between the joins and the fuel inlet. Poke it all around and in the end it came off. But as above, that can crack the end. For bigger tanks this method won't work (I tried).