I bought a big pile of fiat 127 1050cc parts at fall 2005 and also got many 4 gear gearboxes.
One of them had been used in an F-group rally car and had a welded lock and modified gear ratios. You can't drive a weld-lock front wheel drive car in the streets so we had to dismantle the box and change a normal final gear to it.
I was told that one of the gearboxes I bought would of been from sport model and it would have a better final ratio. But all the final ratios were the same so we just picked the best one and installed it with new bearings and such.
This was a difficult job for none of us had opened a gearbox before... all those tiny little parts and gears..

Result: The first attempt failed and you couldn't get the first gear in at all so we had to start all over again.
Now we were more careful assembling all the stuff and I don't know what we did wrong the first time but now it works well..