Of course if the owner/shop did a good job of making the car a manual at some point in its life its going to be difficult to determine since all of the 4 speed and automatic clues will be there. Is this a complete assembled car or something else? Hoping that they may have overlooked something or cut a corner leaving a clue.
Of course if this is a stripped car that was never converted then it makes the task much easier. A little more info on current condition and setup would be helpful
Welcome to the site. Hope you find the information and support helpful in your related restoration or maintenance and original needs as well as tasks
And lastly I'm guessing its not a NJ built K code. Consider adding when and where your car was built - we're going to be asking for that ALLOT of you don't include it somewhere with questions