Rob -- that looks very good. It's tastefully done and adds a sporty look. Looks like factory equipment to me.
The spoiler is a copy of a factory piece used on the 1976 Andretti edition Alfetta GT's as well as some others. There was another version made for the Veloccisma cars in 1978-9. Both will appear on eBay from time to time. Either finish the cars very well IMO, a much better look than any wing.