I managed to fit 4s1p A123 in the stock chassis before, but it was just a mess.
Both FLM chassis are great, but it's easier to fit 3s2p A123 on the extended chassis. Now how you installed them is up to you. I just installed them towards the front right now to eliminate wheelies.
Many thanks for sharing your experience. I'm going to get an extended aluminium chassis and run two 3S1P packs in parallel
it looks like Maxamps have the packs, but they don't tell which formation they are using, bummer! Anyone knows if they are in a flat formation like in snellemin's pictures?