A 270 does use the same case as a 30-06 so the 30-06 can be necked down for a 270......but my question would be why??? You gain absolutely nothing by doing this. The only reason to neck a case down for a different caliber is if you are loading for some wildcat caliber where factory brass is not available.

As common as 270win is you can get factory brass for that caliber relatively cheaply.