Trying 60Hz on a 400Hz equipment is downright ... well wrong. You can damage it! Most of the tech uses transformers in the power supplies, that will saturate at low frequency and burn out.
If I was to build a frequency converter for this purpose, I'd just use a PWM modulated H-bridge, with suitable low pass filtering network. I'd say that there is nothing that fancy in such frequency converter. The more intricate part is to make it robust (short circuit, overload, etc) and EMC compliant, along with appropriate output signal clarity (distortion, voltage precision, output impedance,..)