14.2 second 1/3 mile? (oval)

[MetalMan]

You kinda have to choose between 5s Lipo and 6s. That's 18.5v vs. 22.2v. 5s Lipo would be easier to do since there are more (cheaper) 5s balancers out there. Here's a balancer I just ordered last night:
http://home.comcast.net/~truerc/Charge/chargers.htm (Smart Balancer and Guard 505). At $30 that can do 5s Lipo with a 300ma drain rate, it's a steal. However, you can only charge at up to 3amps when the balancer is connected to the charger, so charging at a higher rate will need to be done with the charger hooked up to the battery, and the balancer hooked up to the taps on the battery.

True RC has some batteries that would be possible choices (and they're really inexpensive). Look here:
http://home.comcast.net/~truerc/custompack.htm
The last battery on the page would work well for you. The weight of that battery is about 200grams less than 16 NiMh cells. Although it says that the batteries aren't in stock, they have the cells, so you might just need to email them about making the 5s 8000mah pack.