Thanks guys. Two of the best things about the people on this site are the encouragement and the sheer knowledge you folks possess.
Kitnut617 sent me some fantastic "how-to" advice which I'm hoping he won't mind me sharing here on the forum. In fact, his pics and text should be turned into a full-blown article (I'd be happy to submit this on his behalf as long as he's okay with it). His method is really the only way this will stand a chance of success, so I think it's time I pull out that great little Matchbox kit and use it instead of the Airfix one I've already butchered (incorrectly).
Here's his explanation:
Keep in mind I don't have a Fortress in front of me so I can't see what the fuselage looks like in the plan (top) view, so from memory I think the fuselage between the two lines you have drawn is not as tapered as in the profile (side) view. You may need to modify what I'm going to suggest to best suit this, but I would maybe cut the fuselage at your left line (cut #1) first. Cut #2 would be a 'Z' cut (try to keep the widest point of the fuselage as far forward as possible) Cut #3 would go horizontal for the same distance as the horizontal part in cut #2 first, then vertically to the crease line of the turtle deck and then along the crease line of the turtle deck. This would be so you can slide the rear fuselage under it when you come to re-join the two halves. You'll notice I've drawn in some triangles, this is so you can swedge down the fuselage to match the smaller rear fuselage, but do this in small amounts at a time, starting with just the width of your saw blade and then gradually making the triangle mouths wider until it can be massaged into shape. Note: the horizontal cut should be on the widest point of the fuselage when looking in the plan view. The turtle deck is the fairing from the front windshield back along the top of the fuselage. BUT BEFORE YOU START, KEEP IN MIND OF THIS:
You'll have to have a good look at the fuselage from both directions (plan & profile) to really determine where Cut #1 has to start and actually look like. It may have to be a smaller 'Z' cut, say starting vertically down just at the end of the turtle deck to my horizontal line then proceeding along horizontally until you get to your rear most line (on the right near the large window) and then cut the rest of the 1st (left) vertical line where you show it. What you are looking for is somewhere on the fuselage when looking in the plan view, where the fuselage is almost the same diameter.