Let's block out some smaller props such as the wings and turrets, so we can finalize the overall placement:
1. Append a sphere. Duplicate and place it six times to determine the placements of the turrets.
2. If needed, use the Transpose tool to lengthen the body, as shown in the next image. Just mask off the rear part. You may also need to use the ClipCurve brush to correct the stretched shape a bit:
3. For the rear air inlets next to the engines, append a cube to the model. Scale and place it, as shown in the next image. Then clip it into shape from the front view:
4. Pick the MaskCurve brush and mask the inlet, as shown in the following image. Then offset it by 2 on the respective axis, so that the unmasked part is pushed inwards:
5. Let's continue with the front. Append another sphere and clip it with the ClipCircleCenter brush, to get a cylinder with rounded caps, as shown in the next screenshot. Flatten and place it at the front...