Graphixv - There's quite a bit of speculation about which 5th gear will swap over, but it seems we aren't lacking in choices. From what I can gather, the 5th from a 1st Gen Probe GT is 0.711, while the 5th from a mid '90s era MX-6, 626 and Probe SE are 0.717. Someone even said that the 5th from an early '90s Fiesta was 0.68, but I'm not sure about that one.
Here are a couple of links about the swap, the second one is a full tranny tear down, and shows how you could swap the final drive also (if you're that brave).
MX-3.com - View topic - 5th gear replacement (F-series)
MX-3.com - View topic - Phantom Grip and 5th gear swap.. B6T trans
Good luck!