Scaling up a hellbrute wouldn't be very hard at all if you cute out the fleshy middle part of the torso and lenghtened that. Then cute out the fleshy bits of the legs and make them longer and thicker. And youd definetly have to give it new arms get rid of that little stubby multi melta.
Hope this has helped.
Also I will probably end up trying this to as it gives me a reason to get another hellbrute.
Also here is a size comparison of the contemptor compared to normal dreadnauts and dreadknight