UPDATE: I went with a Die Hard Advanced Gold battery and it fits in just fine. DIY is a little rough if you are by yourself as the batteries weigh a ton, and there's not much room. Especially with the wires and cables.
IMPORTANT: Just got off the phone with Bavarian Technic. You CANNOT reset the adaptation on M3's (and diesels). Kind of dissapointing because I listened to members and they never even tested this on M3's. You don't have the Command function if you pay attention. No ETA on when or if this will ever be implemented.
Current: 2008 BMW M3 Sparkling Graphite/Bamboo Beige/6MT. 1999 BMW M3 Techno Violet/Dove/5MT. On the list: BMW M2 Long Beach Blue/Black/6MT