I think you should check your physical Model again for the Assault Cannon structure, ... The support that runs under the barrels should be alot wider to accomodate the (missing) Muzzle that should be on the front of it .... even though it's structure is a little different than the Land Speeder version, the principle is the same .... right now, that Assault Cannon would shoot straight through the support
....
Ive got a few ref pics of the Model Kit (preping for mine), and noticed that part missing when I saw your update ... Just a heads-up thats all, ...
Jas