I'm selling a platinum solitaire setting that previously held a 6.03mm stone -- I imagine a skilled jeweler could set your 6.4mm stone without much trouble. It's currently a size 4.75, so sizing it down to 3.25 shouldn't be a problem either.
I also have a platinum mount on DB (reproduction fishtail) that can accommodate a 6.4mm stone easily. I'm not entirely sure how much it can be sized. It came from LJN as a size 6 and has never been sized, but I can find out if you are interested, just message me.