*From a practical standpoint,I think you're looking at using whatever plan taper Ford put in the Escort and blend from there.
*As to the upper radii,you could do as Jaray did in 1922.Take your more rectangular cross-section,and gently morph it into a more organic half-body cross-section as the body proceeds rearward.HVAC sheetmetal shops do this all the time in commercial/industrial ductwork when the dust must enter a chase or wall for part of the run.You can't do it with plumbing due to the pressure balooning the flat walls,but with air,it's a snap!
*The tumblehome can progressively increase as Honda does with their FIT,and the edge radii can also grow incrementally as she goes further rearwards.I worked this into the flying buttresses on the CRX to pretty good effect.
