The Stallion and 416 will fit lipos. The 415 does not because of the way the rear bulkhead was designed to hug round nimh cells. You could try the stick-pack conversion for the MSXX-MRE, but then your looking at the same price as a 416 which is better all around...
The Stallion is not as good for the money as there are still many hop-ups after you buy the car. I would only recommend it for die-hard Kyosho fans. There is a newer "Shin" version, but I've been told it's VERY expensive...