I'm going to be building a homebrew antenna that I will mount internally. My dad who is a ham radio guy tells me there are ways to tune the antenna to compensate for nearby metal that causes interference. So if you know a ham radio guy he might be helpful.
If it were my plane I'd wire up the radio move the fuselage outside and have a friend with a radio go a certain distance and test it to see how it worked. Rotate the plane to see if the signal was stronger in different directions. Probably need to have the wings and tail on it to make it an accurate test.