Ok, you know how you zoom into the fuselage on accident. So go to the place where you want to put the part then you see the next fuselage then put the block I recommend that you use it as a subassembly. It won't fall out it will stay there. I'll make a video today and upload it tomorrow for a better view. A great example is my newest plane the SBD-Dauntless I did this with the fuselage inlet so make sure to go check out!