Where are you getting this 240VAC 60Hz?

I'm going to presume this is for bench testing, not installation into an aircraft.
You can try a low-frequency sine inverter (the kind with a large transformer) or your amplifier with transformer, but for the power levels you are looking for, your efficiency and load regulation will be terrible unless you can get an actual 400Hz transformer--and those are going to not be priced to your liking. Some transformers are designed to work on 50-400 Hz, but those are typically low power.
Repurposing a HF switching UPS inverter might seem a better solution, but I don't know if you can tolerate the noise and distortion that these typically have.
One solution that might work pretty well is to use a bank of individual audio amplifiers, each running on their own isolating transformer, in series. 4 of them, putting out 30VAC RMS (which would be a fairly powerful amp) would do it. You would need to isolate the inputs of the amps, but 4 small audio transformers or linear optoisolaters would do that for you.
Are you sure you can't just buy something? How cheaply are you trying to get by?