If you go to a road long cage der. you can use a 30t cog on a 12-27 cassette. To make the change you will need a 13t first position 8pd cog. You remoive the 12 and 13 and use the spacer on the 30. Because the 13 and the 30 are at the ends of the travel the 8sp cogs will work fine.
Replace the 39 with a 38t ring for a little lower. That will give you a 34" low gear.