The pipe is wanting to "un roll" thats why it popped up like that. You might be able to play around with the ends since you still dont have them capped. Put a bottle jack in the end and see if spreading the end pushes it back down. Not saying it will, but worth a try.
I think I would go ahead and get my hinges on it first and see what it looks like at that point. What type of hinges are you going to use? If you use some heavy material for your flanges you may be able to take care of it easier than you think. If you can get it where you have constant pressure on it and heat it up it may return to where you want it and stay as it cools. Just a thought.
I saw the post about the tank end caps. Starbarg.com had a picture of them for sale. No idea how much, what size. Just saw the photo while searching for metal.
Glad to see ya back at it. Looking good. Send me a message when it's time to move it; I have a skid steer.
Good Luck
Keith