I have a mk5b with a 2litre zetec, its gets used as a track car and I am still using the original 1.6 zetec box its called an iB5 gearbox and supposed to be good for about 200bhp, never had any problems with the gearbox or diff! But if you want to upgrade you can fit an MTX75 box from a mondeo or RS2000/130bhp 1.8 escort or a B5 gearbox from a S2.