Well, my idea is that we could have three parts: A razorback adapter, a razorback extender and a razorback end that could function as a tail mount. I also tried out the prop engines, they are a little unbalanced, but still fun to use I recommend that you change the ISP to a higher value, like 5'000 on the Marlin and 8'000 on the small one (forgot its name). I don't know if that's supposed to be there or not, but the Marlin starts off with a really high thrust value (about 150kn) and drops off slowly to 50kn. Isn't it supposed to be the other way round?
EDIT:
Wait, I forgot something. The half length fuselage holds only 50 units, but it is only logical for it to hold 200 units.