I later made some more modifications by adding gear diff's from Tamiya 416 in front and rear.
For me to get them to fit, I needed to use different belts because the 416 gear diff is larger.
Original S400 has 35t diff pulley and 16t centre pulley.
My modified S400 has 37t Tamiya Gear Diff, and 18t centre pulleys.
The 18t centre pulley are from 3racing 3PY/18.
Original S400 belts are 177mm / 59t rear, and 501 / 167t front.
New belts used are Xray 305444 183mm / 61t rear, and Cyclone HB 70738 507mm / 169t front.
I also need to use a lot of shims because the new diffs are narrower than the S400 blocks. Especially on the front it was difficult, and you can see shims in the photo and also how far the bearing is out of the bearing holder (I have shim inside the bearing holder too).
I enjoyed modifying the S400, and I was reasonably competitive with it. It seemed to be a very solid car and I did not break many parts. However I decided to retire it and bought a S411 2.0, so this chassis is no longer being run regularly. I have a lot of parts for it though and I think I might set it up for stock class if I every want to run in anouther class.