If I remember correctly, 5th gear from a '88-'92 MX-6/626 non-turbo/'89-'92 Probe GL is a 0.700 gear. You'll need both gears under the 5th gear cover on the trans. I did the swap to my '94 Protege and will be doing it to my KL P5. My brother did the swap to his '88 MX-6 GT. It should work in any G-series transmission. The Festiva 5th gear is even taller but requires trimming of the case.
I swapped mine just by removing the splash shield and the 5th gear cover. You need a 27mm and 32mm socket or wrench for each shaft. You also need a punch to get a pin to release the gears/synchros. It's been FOREVER since I've raided junkyards for these gears but I've pulled a bunch of them. My '94 Protege's stock 5th is a 0.814 gear. Quite a nice difference!
Good to know. I will have to start shopping around.