I will change my gearbox and diferencial for have more top speed, and better perfomance for my turbo engine. I will make a combination of parts of stock Vauxhall gearbox here in Brasil. I found this option, and i would like listen good tips for it:

1- 3,58:1
2- 1,87:1
3- 1,23:1
4- 0,92:1
5- 0,74:1
diferencial- 3,55:1

I change the gears around 6.400rpm. I would like top speed around 230km/h, and 0 to 60 MPH in 6.2 seconds, and its in second gear. Maybe this combination can do it?