If the FACTORY valve covers had long oil filler necks it was most likely a 421. 389 valve covers used oil filler caps which were twisted onto the valve covers, 421's had a neck and the oil filler cap pushed into the neck. From the sounds of it you dont still have the car, do you know where it went?

1964 was the first year you could get the single small chrome air cleaners for tri-power setups. Prior to this it was the large dual snorkel air cleaner. Chrome valve covers were also an option in 64 for both the 389 and 421. If the person who ordered the car in 64 sprung for chrome air cleaners it would make sense that they also ordered the chrome valve covers. The only way to know for sure if it was a 389 or 421 would be to run the engine code or block code. I bet the car had plenty of get-up-and-go however!!