This is Phil's current version. See how the sides of the cap step in from the side of the cab? That sounds just like what you're looking for. Also, see at the taillights - the step in there works, since the shape farther back comes back to meet the ideal taper form.
Rather than a low angle all the way back, I'd do a fairly flat top, out to about halfway back, then from there follow the standard ~12 degree taper.
