thats a large cube to do in one time? mostly when you want to cast "big" objects you have to do it in small layers (max 1cm at the time)the exothermal reaction of some resins can build up quit some heat and the more you make at once the faster it will cure and the hotter it will get...
causing some cracks and 'burned' resin...
I mostly use PU casting resin and have already added some graphite powder in it and worked out perfectly