The original tubes are mild steel similar to that used for brake and fuel lines except 1/4" dia tube. Because of the "log" style cast iron manifolds used , the tube would be very close to one for a '65 Thunderbird or Galaxie offering which used the log style manifold. It would not be the same as the production 427 tube which used "header looking" , long manifolds. I am not aware of a reproduction but there may be.
Randy