I’ve had the same question as I’m waiting for my freezer to be delivered before proceeding…
From the best I can tell, piece #28(top middle spacer), as shown on page 18, is designed to be the middle cross support for the bar top and looks to be spaced right in the center. As such, the left keg box frame will be placed directly in the center with equal spacing between the left and right sides of the bar framing so that you can attach the spacer on top of that. In any case, this seems to make the most sense from a structural standpoint. If this turns out to be wrong, it will be quite simple to just measure and adjust the few steps following, that would be affected (ie. insulation panels, front corner frame size, etc.).