Hey Massimo,
It was probably correctable before I glued it all together, but as I failed to notice it, the only way I've figured it could be fixed now would be to saw the tail off, with a perfectly perpendicular cut, glued it on straight, and then add lots of putty, and sand down.
As you can see in this photo, from an angle, it doesn't look too bad:
But viewed from head on, the problem is obvious:
And viewed from the rear:
There is a bit of a warp and a twist going on there. Prior to assembling, it was probably correctable. But I somehow didn't notice it, as both fuselage halves were twisted and warped. Oddly, the fit was perfect...
I've put it aside for now. As I build to help raise money for some family medical costs, I can't take the time to correct it... I just move on to the next build!