A lot of people messaged me about how I did the mod. I don't have time to do make the instruction but I can show you some close ups. Take your Hulkbuster 76104 out and compare with my model, then you will know how easy to DIY. Basically, I just rearranged the bricks in this mod.
1) I shortened the body by one stud. The golden part was taken out
2) The legs were in one piece at L-Shape before. You can just take them apart into two parts. I use two small joints in one leg but there is a better way to do it... I will come back to work on it when I have time.
3) The most important thing is the upper body width. I extended each side by one stud. I also replace the original black arm armor with deep red one.
If you have question, please let me know. I will try to check the comments more often now.