Hi Antonio,
Welcome to the wonderful world of WWI modeling!
I'm also in the process of building that same kit and if I may, I'd like to offer a bit of advice. By the way, what you've done so far looks very good!
Something I noticed in your picture with all of the parts laid out, in particular, the upper fuselage deck/cowling piece - it appears that you have not removed the two ejector pin nubs on the inside, right at the edge of where the metal and wood sections meet. You may have discovered this by now but if not, you will find that they interfere with the fit as that is right where that part meets the upper edge of the firewall. So, grind them down flush with the inside and don't worry about damaging the paint work...you can't see that area once the fuselage is all together.
Another thing, I also replaced the extended gun barrels with metal tubing and found it easier to first glue the gun breaches to the back of the firewall and then install the barrels after the engine is in place. The barrel on the left side actually goes between the intake manifold pipes and the engine block. This picture should clarify the matter:
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I look forward to seeing your progress. You're off to a great start!
Cheers,
Eric