[Stiglr]

I'm assuming here that there is just one "fuselage"....

1) Select that fuselage and make a copy of it; rename it "interior".
2) Make both your original fuselage and your new "interior" ONE-SIDED objects.
3) Select your interior copy, and flip the normals/surfaces so that they face into the aircraft. The original fuselage's normals should face outside the aircraft.
4) For your interior, select the parts of it that are not visible from the cockpit (usually, anything forward of the front firewall, anything rearward of the back firewall or seat back, anything lower than the cockpit deck) and delete them.
5) Size your interior to be slightly smaller along the height and width dimensions (95 - 98%)
6) Work with your interior walls to create the top ledges, and any horizontal or vertical framework.
7) Extrude the various framework to create the interior shell.
8) Create a deck, according to your photos or plan.
9) Likewise select the interior of the canopy framing and add dimension to those (usually much thinner than your cockpit wall framing; about 0.08m thick or so; but depends on the era of the aircraft; modern jet structures tend to be quite a bit thicker than WWII framing)

Obviously, you can now texture your interior without affecting the outer skin.