Have you been on Endless Sphere? That's the place to go if you looking for the gritty details of ebikes.
You'll want to do groups of 5 parallel cells in series with each other. Makes balancing/protection easier and I believe there are several other benefits that I can't think of right now. On a related note, "48v" LiFePO4 batteries are 16s, not 15s. Helps keep you from hitting the low voltage cutoff of the controller if you're drawing a lot of power (probably not an issue with such a big pack), gives you more watt hours, and keeps the voltage higher throughout the discharge, allowing more power.
From my experience with my ebike and a ~16lb 48v 15Ah LiFePO4 battery, I highly recommend not mounting the battery to a rear rack. All that weight plus the weight of the motor back there makes for a bike that handles like garbage. If you decide to use a rear rack anyway, I would put half of the battery on each side as low as possible and do whatever you can to make the rack stiff.
Here's my ebike's build thread:
http://ecomodder.com/forum/showthrea...ike-28487.html