I took a piece of 5/16" drill rod, which measures .3125" diameter, and made a long tapered expanding "ball" on my lathe. I put this into my .30/06 sizing die to neck .270 Winchester cases up to .3125". I then run the necked-up cases into my .280 sizing die to create the false shoulder on the neck.
This approach gives both the correct case length and also gives plenty of area for a good false shoulder.
However, since .280 brass is available from Remington and Winchester, I'd prefer to use factory brass since it has the correct head stamp.