I don't think that is a good idea, the gearboxes are related to the power and torque of the engine.
Changing the gearbox (with other ratios) could damage your car and probably won't make it drive better or faster...
but then, i am not an engineer. i could be wrong..
I'm on my second Bravo 1.6 and on the first one i had this figures:
5 gear 5000 r.p.m. 170 Kph
Top speed on speedometer 220 at red line.
Now it makes less 800 r.p.m. for the same speed and althougth the consumption is better i use the 5 gear only for fuel saving. 3 and 4 gear are also longer. Fiat says that the cars are equal, but either the gearbox or the diferential must be diferent.
Lois