I have recently bought a copy of the Longstreet ACW rules. I know these are not Old School but after a couple of games I can confirm they are a cracking set that are great fun to play. It's been while since my ACW collection has been on the table however we are planning to kick off a Longstreet campaign in the next few weeks. So you can expect more updates on this.
Here are a few photos of one of these games.