The Script Blocks don't have to 'Act As Pressure Plate' to be activated when the chest is opened/closed, the chest will emit power while open, so as long as the script block is placed adjacent to the chest, it should be activated when opened.
As for the script, it appears to be written correctly, just be sure that your coordinates are correct. An easy way to do this is to use relative coordinates, which also allows you to use the same script anywhere throughout the map. Typically, underneath of a chest is a good place for the script block, so for that location you would use the coordinates [rel:0,1,0].
Another way to do this would be to have a SetBlock command place another chest in place of the one with the items in it, essentially just replacing the full chest with an empty one. For this method, if using the power emitted by the chest to activate the script, I would advise putting the script in the 'Powered off' section.