Hi Ben, what's wrong with you current gearbox, and what are your future plans for the car?
They are a very strong box from the factory (not many cars come standard with a box that handles 2-3x factory power) so what you have may not need anything, particularly if it doesn't crunch on quick changes from 4th to 5th
If on the other hand you are building another 500kw time attack car, you will need something a bit more expensive.....