540 and 500 size motors are significantly bigger than 400 and 380-size motors, and also much more powerful. I would be very surprised if Heng Long gearboxes are up to the challenge, for any long-term applications. However, if it's a short-term project that doesn't need to last for years, it might be possible. Another possibility is to make your own gearbox.

out of curiosity, what sort of project are the gearboxes and motors for?