Create a tk2dCamera, set native resolution to 480x320.
Tick Force editor resolution, and set it to the preview resolution, eg. 480x320 for now. (not REALLY necessary, but makes things a bit easier to understand).
Add ONE override, and leave it at defaults. That will scale any resolution to 480x320 (but will scale the largest axis, leaving either extra information on the width or height).
If you want it to non-uniformly scale to fill the screen at ANY resolution, change the override "Auto Scale" to "Stretch to Fit".
That is it, nothing else should be required.