I just modified one of my 501X to accept the Trakpower 3200mah lipo - was quick and easy. I have the old 501X shells, and I don't think they are high enough for the bigger lipo saddles? so I didn't try to modify the car for those, but 3200mah should be plenty.
to get the cells to fit I just flattened the inside edge of all four posts - the bottom flares out to keep the nimh cells firmly held. I used a sanding drum and it took about 10 minutes from start to finish.
The cell strap would fit upside down (foam facing up) but to get the foam to press on the cells I had to raise the battery posts slightly with a small 1mm Xray washer.