If you only use the front 1/3 of the bar, that's a lot more reasonable, just make sure you mount it strong because that's still a good what, 400g? Sure you already know that though.

Definitely use a bigger battery though, I feel that the one you've suggested won't cut it, both capacity and current-wise. You'll want it to run for a good 4 minutes at the most, I'd say - just to leave yourself some lenience so you'll never run out of battery in a fight. I'd assume you'd be running those motors at full pelt for the full 3 minutes of the fight so that'll require quite a beefy battery I'd guesstimate.