Why not anneal the cases, run them through the fl 280 die as you normally would, seat cheap bullets out to touch the lands (no headspace issue this way) use a slow for bullet weight powder about 1.5grns under max with like cci 200 primers fire them and bam, a brand new batch of 280 cases with a stronger case web to boot, because I bet the 270 brass is thicker and heavier at the case head and up past the web due to the 270 having a much higher pressure rating. To each his own, be safe.