Velineon 3500 motor is only rated for 3s lipo (11.1v)
If you are planning to run Lifepo packs you should be looking for the 3s size (9.9v)
Something like this pack would work well
http://hobbyking.com/hobbyking/store...dProduct=10310
You will need to change the battery connector to match your ESC
I would also suggest you change your ESC battery cutoff voltage to suit the Life pack
Cheers