After cutting apart the original torso, I was left with almost nothing for the belly section, just a thin strip of plastic. So as I did with my Hi-Zack, I scratchbuilt a new midriff. This was less curvy than the Hi-Zacks, so it was simpler to build. I cut the top and bottom shape out of sheet styrene (using measurements taken from the waist and chest to get the dimentions right), then glued styrene strips around the edges like a box. The panel lines were made by actually gluing thin plastic panels over the sides, leaving space in between to make the line. The ribbed part in the back was made with pre-scribed styrene.
Once I had this built, I cut a hole in the top to accomodate the ball joint socket, filled it with Bondo, then squished in the socket for a perfect fit. This new midriff just plugs into the chest, and was measured carefully to allow a little bit of rotation, and also bends side to side (I failed to shape it correctly to bend front to back though).
As I do with a lot of kits, I found that I needed to beef up the groin a little bit (I really need to find a better way to say that...). I find that thickening the center parts of MS (like the middle of the chest and groin) really adds a more dynamic look to the proportions. Anything to get away from that boxy look!
In this case, the simplest way to do what I needed was to glue some styrene strips around the edges, building up a frame the thickness that I needed. These were beveled towards the edges with sandpaper. Then I filled the cavities between the strips with epoxy putty and sanded until I had the contour I wanted. Also note in the pictures that the original midriff had yet to be cut off when I did this.
Just as I did with the chest, I filled the waist parts with Bondo to facilitate casting them later. I also created the surface that connects to the midriff by pressing the scratchbuilt part into the still soft Bondo (turns out to be one of my more usefull tricks).
The pictures below show the difference between the original and bulked up parts. It's a pretty subtle difference in shape, a little skinnier from the front, sticking out more from the side, but the difference will definitely improve the boxy appearance of the original kit.