Or you could use a small 2100-2500 mah 2 cell lipo (its all you need), and run a slow motor like a stock or crawler (Novak 18.5). I heard some oval guys are trying this in California, don't know the result though.
I've played with both single cell voltage and 2 cell voltage.
With single cell, you have to run a hot motor to make it run fairly quick. A 3.5 motor runs about like a 5.5 turn 4 cell car. It's actually prtetty hard on the diff also. This was with a home made pack from 2 maxamps receiver packs (12 cells and about 5000mAh). The speedo works fine on 3.7v but you need to run a receiver pack.
With 2 cell voltage, it's definately mod only. A 13.5 is pretty fast and anything over a 10.5 is going to be sketchy on runtime. Personally, I like running 2 cell voltage because the reduced capacity brings some strategy back into the picture. Having a class that can dump limits the speed a little and brings the racing back into the mix. You can fit about 2700 mAh into the car with a 2 cell Lipo (saddle pack).
The biggest problem with running 12th LiPo is the fact that the weight balance gets all screwed up. The cars pretty much handle like crap unless you put a ton of lead on the back. This pretty much kills the LiPo advantage IMO.