So I decided to make the jump to LiPo's in my Summit. I went with the SPC 8200mAh packs. These are a thicker pack and fit perfectly within the width of the battery bay. You do, however, have to modify a few things to make them fit nice and clean. I decided to drop a thread out here with pics and things I recommend doing before you tear off into the sunset!!! So let's begin.
The first thing you have to do is remove both right and left rear battery vents and trim them up. I am going to run the wires through the vent opening. I am using Traxxas connectors, so I have to remove more than if you are using a different style that is smaller. This can be done very easily with an Exacto knife or razor blade. The pics here show the main webbing removed but the corners still need to be smoothed off. You do not want any sharp edges that could potentially cut into the wiring of the battery.
Right vent with two webs removed.
Left vent with one web removed and one trimmed about half. You could cut the entire second web out if you like.
Now that you have those done, you can reinstall them back onto the chassis.
Next, we remove the battery filler (used for thinner batteries). This piece will tilt down and pop right out.