Hello,
I’m going to try a contour cut of 320 x 350 x 275 mm, which is the max work area for the A350.
Obviously, there will be little room left for clamps in this situation. What’s the recommended way to securely attach the stock to the work area in this scenario? nails? other?
You could try double sided tape. Another trick (haven’t tried myself) is masking tape on your spoil board, masking tape on the back of your stock and super glue them together. You should be able to get it of fairly easy afterwards.
But I am confused by your dimensions. Will you cut 275mm in height? (so 27,5cm?) You either need a ridiculously long milling bit or I your tool head will bump into your work piece, no?
2.75mm of course 
Do you have a few places in your stock, where you won’t be carving? You could drill a 4mm hole there and fix your stock with 2 or 3 M4 Bolts to your Wasteboard.
Print something to hold your board down 