Hmmm .......I bought a Panda 750cc a few years ago and searched for ages to find one with a 5 speed gearbox. Once I had one I realised that I'd wasted my time. The engine just didnt have enough grunt to drive it. You would have to be at full throttle all the time to just maintain a steady speed on the flat. It was always labouring in 5th. You would have to be almost at top speed of 4th before you could select 5th because otherwise the car would just slow down. The car on flat ground would have a higher top speed in 4th. Its only usefulness was for going down long downhill sections on the motorway - so not really worth it all in all.
If your 126 has a standard engine then I would say the standard gearbox ratios are fine. My BIS cruises along in 4th gear at 60 - 65mph on the speedo (55-60mph on GPS) with no fuss at all. Thats with the 1st throttle wide open and the 2nd throttle closed. It doesn't feel like its revving too high at those speeds, but by the same token if I then press the accelerator all the way to the floor there is hardly any more acceleration so I feel that a 5th gear with an even taller ratio would be just too much for it.
If you have a tuned engine with considerably more power than standard and you make almost all of your journeys on motorways then it might be of some use to you. In fact it might yeild a higher top speed, but that brings me on to the next thing - These cars aren't designed to survive high speed crashes - there's not much to protect you - no crumple zones - side impact bars - just some thin steel, a crash at 80mph in a Fiat 126 would much more likely be fatal than larger more modern cars.
Sorry to put a downer on your project, I just wouldn't want you to do all that work and end up dissapointed with it. If you do decide to go ahead then let us know how you get on.