I would avoid cast. I'm not certain about yard and service lines, but cast iron distribution & main lines have to be inspected regularly. Also coupons from the cast must be removed periodically and tested for wall structure integrity. If possible, perhaps the pressure can be increased at the regulator, CSST run using the existing cast as a casing and another reg at the home to reduce to house pressures. We have also used PE as an insert piping in older steel lines by increasing the pressures to accomodate for the smaller diameter. Is PE not an option for you? It is compatible with LPG, NG and very installer friendly. Whatever chosen might require some coordination with the gas supplier and your installer. I am suprized that Cu is allowed in Nebraska. Our gas comes from part of the same network feeding Omaha and Cu fuel gas piping is only permitted for LPG. I wouldn't use cast, period- even if it costs 1/10 what the other choices do. Soil subsidance and other conditions can lead to failures.