Ah ... well, that's a whole different kettle of potatoes. Short answer? You don't. You find an 8" - 9" 4:3 VGA (or better) CRT (or LCD) that fits in the shell and work with that.
Long answer? It *might* just be possible, but I'm afraid I'll have to redirect your attention to Messrs Search and Google. It'll take a reasonable amount of research, and hackery, and I don't at this moment understand it well enough to give you a definite yea or nay.
If the 6400 is capable of outputting 512 x 384 ... that would be a promising start.