I think part of your problem is that you are exporting the "drawing" or "page" (see pic 1). You should select all the objects and then export the "selection" (see pic 2).
However, there is a few pixels difference between your desired output size and the actual size of the graphic. This can be sorted by using sledger's suggestion, for instance.
By the way, there is no need to make your page size the same size as your graphic. Just create your graphic on any page size, group it or select it all and export it as a selection.
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