I used 4000's, but to fit, I would either:
a. Get a weight tray that positions the battery out from the bulkhead.
b. Grind the bulk head ever so slightly, just so the car has room to flex with the pack in it.
For weight, I just used a weight tray and about 74g of lead balancing weight. A combo of 45 cal slugs and some 14g bricks.
RC America|EA Motorsports|ORCA
WCICS Rep - Round 1, Saskatoon, SK