Look at FLM's forward tranny XL chassis; I'd ask about max. size that would fit in the battery area and it was 4S and 5S or 6s was really dependent on size of battery.
Not sure how Jahay is stuffing 8-10S in his Savage, but I'm having enough issues putting 6S in my 5th scale conversion as I move to a center diff and thats only using a 1520 can.