Hello Stellan...
Looks great my friend. Now I'm really getting excited to see how it progresses from here !
As far as US Civil registrations, to my best knowledge, the "N" numbers are only required to be on each side the fuselage, and I "think" they are only required to be around 4 to 6 inches tall...
Most are applied just under the stabilizer on Warbirds. This way they are not too noticeable...
Like this picture I took of a P-51 recently: This one has the "N" number of NL5427V
Many years ago (back in the 1930's and 1940's) they were placed on the wings too, and had to be much larger in size. One set of registration numbers was visible from the top of the wing, and one set visible from underneath. The older style "NC" upper registration number was on the Starboard upper side, while the lower was on the Port underside of the wings.
Back then they also started with the letters "NC", where now they simply use the alphabet letter "N".
The aircraft registration numbers for the US always start with an "N", but then various combinations of letters and numbers can be used...
Hope this helps...