Position a crop box at 120 px and 500 px on the x and y axis

Wouldn’t using Image > Canvas Size twice work just as well? 

* Double click on the upper left intersection point of the X & Y rulers, to reset the origin point to zero (if it was elsewhere).

* Double click on an X or Y ruler to bring up the ruler preferences, set the ruler to PIXELS as the unit.

* Bring up the info panel (F8) window

* Move the crop tool or the rectangular marquee (selection) tool to 120 px and 500 px and ensure that the width and height is 500 px.

Holding down the spacebar allows you to reposition as you "draw", then release to continue "drawing".

Another method is to simply draw a marquee selection at any size/position, then press CMD/CTRL T to transform the selection and set the anchor point to the upper left and then type in the X + Y + Width + Height. If the position/dimension fields are not set to pixels, you can contextual/right click in the field and change the unit to pixels or just type in say 500px to override the current units. Then use the Image > Crop menu command.

Edit: I once created an interactive crop script to crop to a desired square size here:

https://community.adobe.com/t5/photoshop-ecosystem-discussions/how-to-lock-canvas-resizing/m-p/12634283

 

So you would type in 500 and the crop box would be centered on the canvas, the script has the selection transform active, so then all you need to do is type in the X and Y position and press the tick or hit OK and the crop is then performed. An action could do the same.