Your axle inclination will dictate your spring and crossmember. If you want 6 deg in your axle, your spring and crossmeber will also be at 6deg, all parallel. You can change your wishbones to fit your frame , common is to pie cut it near the factory weld at the front, put the rear where you need it and weld it up...well. You will always have a little caster change due to the axle swinging on an arc, so longer wishbones the better. Only way to get rid of this is to have a 4 bar or infinitely long wishbones. It is quite often I see a crossmember welded in the frame flat, after kicking your 6deg in for caster, raking the car a bit, all the sudden you have 10 to 15 deg of miss alignment...bad for poly bushings...