Acrobat Pro DC custom Preflight profile to add set sized background.

I have a PDF for example that is 50x50mm.

I need to scale said pdf up to 340x340mm and add a hairline. Then, I need to have a white box behind it at 440x400mm to "frame" the content of the pdf. Now all of this is easily done manually however I have a couple thousand pdfs to do this to. (eeek!!) Some pdfs I will be processing will not be square (eg - 50x100mm) but they need to be scaled up with the same principle and again placed in the centre of a 440x440mm box.

I can scale the pdf to 340x340mm for both square and non square items, that part works... (with the following fix up setting)

Scale pages
fit from inside (scale page edges proportionally)

and I can add the hairline with Outline page geometry boxes

Now the next stage has been kicking my ass, I cannot seem to fathom how to set the page dimensions to 440x440mm and fill with white with success.

I've attempted to Set page geometry boxes and then follow up with a Add white background to no avail with several different combination on the settings. Is this possible? Have I blinded my self from the obvious in frustration because nothing seems to work..

regards,

Phil