Just as everyone confirmed, the part#'s are different for the manual & the automatic. The giant label on the ECU will indicate if it's a manual tranny, or a automatic, thus the reason why I questioned as to what tranny you have.
One of the minor little differences, is the way the ECU controls the fuel management, when the tranny is shifting gears. Esp on the automatic. Thus one of the main reasons the ECU's are labeled differently & the different part numbers.
Auto ECU will behave differently on a manual, but it will be so subtle, you won't really notice it. Peeps have done this already & it's fine.
84 ECU SHOULD be okay on a 83. It should not have anything weird, like on the 82 w/ the vacuum advance distributor, or the different electronics on the AFM. Or like a 85 - 86, where the ECU will be looking for a knock sensor.