My basic approach was to saw off one or two arms and reposition then either as they are or extend them. Especially the power fist sticks out, so I wanted to move that. To get some freedom, I extended the arm using some pieces of sprue and then filled in the void with green stuff.
I bulked up the green stuff in there to something vaguely beefcake like. A daemon prince shoulder armor pad fits on top nicely. Here is a rear view of the modded model next to a regular one.
Messier than the arms are the legs. They are connected only with some strands of plastic, which are the leftover bits of the original cybot hips plus the daemon flesh growing over and around it. To modify the legs I cut them off and then capped them with plasticard, which I then trimmed to the shape of the piece as well as I reasonably could. That gave me to flat surfaces that I could angle as desired and get a very strong glue bond. This model here is in full run while shooting and ends up running on three limbs. I hand-cut a Khorne icon from plasticard for the gauntlet and had just glued it in place when I found back one from the Chaos Rhino spure. Ah well! =]
Here are the four brutes side by side. From left to right the original, arm moved, arm and legs moved, arm extended and position changed to slip a casualty in.
I mounted two 4mm round magnets into each brutes right wrist to be able to swap the weapons as needed. The laser cannons on one of them are too big of course, but I didn't have more of the smaller ones at the time.