Using the second intermediate frequency, often at 455kHz, is widely known. A suitable I/Q-SDR would be based on a 1.8432MHz (canned) oscillator, resulting in a 460.8kHz center frequency. Assuming a 24k sample rate, would be adequate to cover the range from 450 to 460kHz perfectly.
Many I/Q-SDR kist are available, due to the size and the low price, the softrock lite could be suitable best.
So, why the ATS-404. I was looking for a relatively cheap wide coverage receiver having direct frequency entry. The 5kHz tuning steps on shortwave suit the 10kHz wide SDR just fine.
The ATS-404 uses the TA8132AN AM/FM-receiver IC. This IC has go the advantage of providing an IF-out at pin 9 (see data-sheet). Slight downside: the TA8132 employs a 450kHz intermediate frequency...
A service manual for the ATS-404 can be found here:
The remaining question... where to put the I/Q-SDR? Using a softrock lite, one may consider using the battery compartment. Speaking of battery, the radio runs of 4 AA cells. It can also be operated from 6V external power. I wonder if 5V from a USB-port would be sufficient, finally, the idea is to use a computer for the SDR anyway.
Which brings me to the last idea... a cheap (stereo!) USB-sound-interface could also be accomodated in the battery compartment, so that only connection would be a USB cable to the computer.