You are correct that American and Northwest were the two carriers operating on New York to Detroit in 1964, both with 707/720s. If the recollection was of a notably silver aircraft I would definitely think of American first, with their classic all-over unpainted polished skin and orange lightning-flash. This was apparently significantly buffed up to shine, at some considerable ongoing cost to American, so on a brand-new aircraft would really stand out in memory. There was a sad "heritage" reincarnation of the livery done on an American 737-800 a few years ago, with dull grey paint (not even silver), and missing several of the orange flash details.
Boeing_707-123B_American_Airlines_JP6855539.jpg (1024×696) (thisdayinaviation.com)
Northwest always had the all-red tailfin, but the rest was a dark blue cheatline and conventional white, just unpainted on the lower belly.
First flight of the aircraft ? American had their HQ in New York then, it was only later they moved over to Dallas, so a likely delivery point. Northwest was always centred on Minneapolis, HQ and maintenance, so less likely to have an aircraft first flight starting at New York. Both carriers were getting new aircraft that year.