Plus, it's pointless, because unless you're using SuperHires, the Amiga doesn't display any more pixels than that, including the full overscan. And none of the UAE releases I've seen correctly support 35ns pixels (it treats them like the A3000 scandoubler, skipping every other column).
I recommend either going with 800x600 or 720x480 (NTSC)/720x576 (PAL), if you can set the latter, or anything with a width close to 720. Anything larger and you're just wasting space with 99% of games, since they use the custom chips for display and audio.
If you're running P96 compatible stuff also, I recommend just using the largest resolution that you can easily read that the XBox supports. These tips do also apply to standard UAE as well.