Although you can mount both oddessey 680 batteries on the firewall, it presents many challenges in doing any work behind the engine and I had a very forward CG. I now have one on the firewall and another back by the hyd pump in the empanage. My CG is very good now and I picked up about 7 knots after the move.

My advice is to hang the engine, install the panel and weigh the airplane to determine the best location. There is so much you learn after finishing an airplane. Best locations I have discovered talking to other builders with good CGs include:
-One on firewall, one in empanage/hyp pump in empanage.
-Both small batteries behind seats//hyp pump in empanage.
-One large battery behind seat/hyp pump behind seat

Hope this helps. Also, for Legacy builders, you can read my site or contribute to the Legacy builder's forum:

http://www.lancairlegacy.com/
http://www.highrf.com/forums/


Don