ALL 5th gens came with stamped/formed pans from the factory. The steel pan is much more forgiving than a cast one. If you insist on having the cast one-take it to a certified welding shop that can do TIG on botht he inside and outside.
Another thing-make sure that the shape of the 2 pans is identical. It is possible that the cast one will require a different pickup for the oil pump. This is the case with Moroso pans, which are designed more for the track than street use.