Just to add to Greg's comments, you won't be able to simply use the car packs as-is. There will be some reconfiguring involved, combining two 6 cell packs into a single 10 cell pack, with two left over of course.

The "take a car's power system and put it in a plane" method is why many old-timers still scoff at the notion of electric power. They remember the old GP ElectriCubs and Goldberg Mirage 550's and Electras that flew like absolute CRAP on a Speed 600 motor and a 6-cell car pack. To get adequate power from a 6 cell car pack, you had to draw lots of Amps. That meant HOT motor, HOT batteries, and flight times shorter than... Well, I don't want to be conjuring up mental images so I won't go there

As a low-cost solution, I would highly recommend Gary Wright's E3D power system on most any plane in the 3lb range designed to fly on a .25 glow engine. Heck, I'm using it to power a 5lb .40-size trainer.