I would start with a bunch of spheres, then use Scale or deform the control points to mass the basic form. Try to hide as many of your surface seams possible inside the mass, so they will be absorbed when you boolean them together. Extend the back end much further than you need, as you can easily trim away the excess later with a boolean difference using a box or a curve. Use trim or boolean difference to make the cutouts for your engines...

jonah