Well, I have been very hard at work designing and constructing the gun mount. I don't have plans with any measurements just photos from the net. I am trying to make it look as accurate as possible from the top. It also helps that now I have a photo from underneath the gun too. I now know how the mount sits and traverses. Huge help in understanding how to replicate that.
Here are some photos of my gun mount so far. Its not finished yet, some parts are taped together since I haven't installed any electrics into it yet and still need access into the gun mount for that.
Lots of planning, test fabricating and frustration have led to this point where I am satisfied I have done the best that I can do to replicate the appearance of the 75mm mounted in the half track.
Going forward, I may have to fudge the storage boxes in the floor in the back and make them longer than they should be to hide the IBU but that will come in two phases from where I am right now.