It could make it worse trying to straighten it, due to the realtively low curve. Take out both pieces of card, and put some inside, as mentioned, filling later, when set. Make sure the front of each piece of card is vertical to the cockpit rim. You might have to dry fit the fuselage and spine to check this, as you'll need to see where the card actually sits.
It might be an idea to use each side of the spine as a pattern, and trace the shape of the triangular cut-outs onto paper, then transfer this tom the card, allowing an overlap on the inside, so that it can be glued. thsi will also help to achieve the vertical line of each piece of card.
Hope this makes sense. And now, I'm off to try to do a bit on the MiG, then bed!