What were the factory recommendations? I'd stick to the same 100C grade (e.g. 30 or 40) and then the "W" grade would be optional, as oils have really progressed in that area since 1964. The other question would be... "Do you really expect to run this car hard? Or during the winter." Truth be told, most collector cars are garage queens most of the time,and used only in warmish weather, so a a 10W30 or 15W 40 HDEO would be a good choice. They are robust, resist oxidation should do well in a AC application and they have lots of ZDDP, which would suit that engine well.