A TH400 should never be put into neutral. When you does this the transmission spins at 2 times the rate of the engine rpm. They are not designed for that.
However, you can get a "clean neutral" valve body. This is available from most companies. It adds and extra neutral gear that allows the car to safely be shifted into a neutral gear while release the high gear assembly so the transmission does not spin at twice the rate of the engine RPM.