Let's face it; the Mk3 part system is useless.

[CaptainTurbomuffin]

They are definetly not useless. Add a rocket fuel tank into a Mk.3 unit with 240 less liqulid fuel, and both the problem of COM and oxidizer solved. you also got more fuel in it. For the case of dumb weird attachment points, use the BZ-52 Radial Attachment Point.

But note that these are botch solutions. 2 options for the fuelsage should be added:

• one is for carrying fuel, or LFO, or nothing ( structural fuelsage),
  
• one for its type: Lots of fuel, or courrent fuel ammout(with a lot better fuel-mass ratio), and MOST IMPORTANTLY a cargo-bay.
  

For making optimal desgins, there should be another Mk.3 fuelsage with the half of the courrent one's lenght, and this Mk.3 Fuelsage Short should have the mentioned options. The low COM gives more stability for planes, and comes quite handy with shuttles. Adjusting the COT to it would be a bit hard, so still use attachment points for that.