Letterbox scale will work fine, but you will get black bars at the edges on 1136x640 devices. (assuming a landscape app)
To avoid the black edges you need to use 'Scale Outer' or 'Scale Inner'. I would use a 960x640 window for the project, with Save Outer. Make sure the first event is 'Scroll To 568,320' so the window view centers on the layout.
Make sure you have a background image of 1136x640, so the inactive part of the window shows something other than the black bars.
You can't put any interactive content at the outer edges as they will get clipped on 940x640 devices. So from 0 to 88 pixels, and 1048 to 1136 are dead zones, but you can still use this space for some things, e.g. bullet objects coming from the screen edge. It means 1136x640 users may have a slight advantage as they can see things sooner than the 960 wide screen.