Yes, you should be able to build the harness with just the measurments provided, but I wouldn't narrow it down to just two measurements. The more the merrier. To be safe, I would probably sketch out a diagram of the harness and label the measurements, "A", "B", etc. and ask the customer to provide these measurements. I've built harnesses for customers this way. Mine is a little different because the gun sits at the middle of the torso. I asked the customer to provide measurements from the center of the chest over one sholder to the middle of the back and then the dimension around his torso. Like I said ours are different, but I believe you can do this with the dimensions provided plus the adjustability that you are building into the piece.
Hope this helps!