You also need a charger that can charge them and a balancer for the A123 batteries. Are you talking about this battery http://www.battlepack.com/LFP18650P.pdf ? I have heard those aren't true A123 batteries, but they are the same chemistry. A123 batteries only come in a 1100mAh and a 2300mAh cell. You probably need to solder them end to end. Are you planning on assembling them yourself or are you planning on buying them pre-built? If you want them pre-built I can give you a link for a company the builds packs of A123 cells. Then you need a hot soldering iron. There are plenty of people who have used them on T-Rex 450's on Helifreak and RCGroups. Are you planning on discharging them 100%? I am guessing the B400 will use 180mAh/minute so you would only get 6.25 minute flights at 100% discharge.

Nick