Hiya,
We seperated our sow about 5 days before, but if I'd been spot on with my dates it would have only been 3 days
She really didn't look pregnant until about 4 weeks before she was due - I kept having doubts
I gave a bit of extra food towards the end (but not much, just an extra scoop - it's a Kune Kune I've got, you could maybe up it a bit since your sow is bigger) Just keep an eye on her and see if she's looking for extra, you don't want them to get too fat (other than because of the piglets!) or else you could have problems when she farrows.
Her udder will start to develop up to a month before, her vulva will redden and swell a couple of days before and once you can express milk from her, labour should start within 24 hours.
As for your mix - WOW the resultant piglets will be tasty, long bodied and very very cute
Good luck
Karen x