To last an 8min race, the best way it to charge at a low rate. I'd hit those suckers @ 5amps maybe 4.5 if you still can't make time.
I'd also get atleast a total of 3 packs. Then get another charger.
2 chargers and 3 pack = one pack is always ready in the car and the remaining 2 are either discharging or peaking.
What I usually do is charge the nite before and then practice threw each pack, let them cool, discharge then peak for the quals then keeping the best pack for the main.
Another step would be to charge at the track; that would mean getting to the track setup to your pit space discharge 2 packs at a time then charge both at a time. When the first pack peaks pull it off and then start to discharge/peak the next pack. Pratice with that freshly peaked pack, once finished let it and the motor rest. Once either pack or motor is cool start the process over again.