Geoff, your 1965 Flairbird would have left the factory years ago with a Tranny Code of 4 or a 6. Both being a Cruise-O-Matic, but the 4 would be cast iron and a 6 not. Your Data Plate will tell you what the Tranny Code is. Your Vin # should have a Z in it telling you that it was the non-export version of the 390. If it has a 9 for the engine code, then it was an export, low octane engine... It is possible, unless you know the car since it was built, that a long the way a previous owner replaced the engine and the tranny... Here is the Specifications webpage for your Tbird, followed by the main page link. This will tell you a lot more about your 1965 Flairbird.