Looking at 3-Views of the DC-4/6, it does appear to have a slightly oblong cross-section, higher than what it's wide. I think a series of round bulkheads all the way through the fuselage would be enough to make it circular, I'm not going to detail the cabin area anyway. Or if I can find the correct diameter of pipe, a bit of ABS pipe would also work. I had found one particular diameter pipe was just a fraction smaller than the inside diameter of the Airways Vac Form Britannia I'm converting into a CL-44, I think they have the same diameter fuselages in real life. I'd have to check on that though --- it's also something the Contrails B-36 kit I have also does, the vacuform kit came with a length of PVC tube for inside the fuselage to stiffen it up.