Right, first suggestion(s), then see what the guys and Herr Doktor Wojtek have to offer.
Check, by dry fitting, holding in place and Mark 1 eyeball, that the lower centre section of the wing will be OK if glued in place and clamped until set. Depending on the space and angles available, you can 'clamp' with rubber bands, clothing pegs, tape or a combination, 'Bulldog' clips are good if they'll fit.
Once the cente section is glued, set the model on a smooth, flat surface, and check that the angles are OK and equal on both sides. You'll need a vertical reference point, for example, a vertical window frame or similar, that is visible behind the model. Line up the fin with the reference point, and look down the fuselage, from front and back, to ensure that this is 'square'. If it is, then check the angle of dihedral (if there should be one), or level, of the centre section.
Given that you're happy with this, then you can tackle the wing - tops. If you're NOT happy, then look to see what needs doing, and correct if possible. If not possible, post pics and ask again.
Now, if it's OK, now dry fit the wings, again taping, clamping or whatever, to hold them as close as possible to where they should be.
Check the angles again, particularly that each wing is equal, and that the dihedral is correct. It may be that the latter will be out, but the angles in relation to the fuselage, and the distance of the wingtips above the flat surface are what you need to see, in order that the finished model will LOOK right when it's sitting on it's 'legs'.
When you are OK with this, then look at the gaps on the upper wing roots. From what I've seen in the pics, you'll probably need to bridge the gap to a) allow the wings to mate and glue to something, and b) to fill the gap.
I reckon that strips of plastic sheet, or plastic strip or rod, will do the job, or possibly even stretched sprue. Remember the similar problem I had with the '110?
You can trial fit whatever is suitable and available with the wings dry-fitted and taped. Now, assuming you have found the right solution in the form of materials, glue the top wings, trying one at a time, starting from the tip inwards, until you reach what should be the root. If there is some 'spring', or play, near the root, you might have to use a couple of spots of Superglue (CA adhesive), just to help hold the joint until the poly cement sets.
When this is done, then you can insert the strip/rod/sprue, cementing it with liquid poly. When this has set, if neccessary, sand/file back, and then any remaining gap(s) can be filled. If it's a small gap, try the talcum powder and varnish 'porridge' trick I use on my models.
When this has hardened, then any required sanding can be done, and you should have a good, smooth joint, whcih will probably be stronger than the designed original.
This is a problem with the A.M. kits I think, even though the Avenger is my first one. They are made so well, and close, to scale, that any fit problem is a big problem, due to the very small locating point/areas.
I hope this helps Chris, and if you've any probs, PM or e-mail me, and I'll see what I can offer.
It may be that Wayne or Wojtek can come up with an alternative, simpler solution, so I'd advise only doing the trial fit for now.