Yes change the body post, just slightly narrower than the senton ones.
One thing I did and I have seen a few people doing this is changing the wheels to the ones from the granite, also 14 hex. It help stabilise the car to avoid so much rolling. The motor pinion will need to be reduced to 15t (20t on the senton), it will help the motor. By changing to wider wheels the senton body will need trimming to have clearance for those wheels (if running with that body), the big rock one will be just ok.
I have also turned my Senton into a big rock by swapping the arms for the longer version.
Senton and big rock share the same long wheel base chassis, so a few opportunities to improve the car in some ways lol
I did do it mainly for the winter and the muddy fields, in the summer I am looking at reversing back to the senton