A body on a modern car ?

You should probably consider just how unsafe this contraption is likely to be before putting Your Child in it. What You propose is much more involved than just managing to get the body on there. Unibodies are engineered integral structures(tho' imperfect sometimes), torque management, torsional stress, & most importantly impact management are going to be altered substantially. A cool-looking death-trap that gets worse fuel economy than the base platform isn't worth it IMO.