I have had custom exhausts made and never been impressed with the quality, I had a power flow that looked very cheap and used to knock against the bumper, a local company called Custom Chrome have done exhausts for myself, my dad and brother, we had problems with all of them, bad fitting, torn out threads even dented cars.
I would get a ready built one, heard good things about blue flame.
Personally I have a HKS Silent Hi Power back box and love it.