You should get the gearbox internals cryogenicaly frozen mate whilst your at it.. Costs around £120 and increases tensile strength by around 400-600%
Most of the TL4 boxes break following some form of synchro wear/damage -gears themselves seem pretty durable
The cryogenic treatment I'm not a fan of as it produces conflicting results, also to quote machinery's handbook: "subzero treatment may sometimes cause cracking". Nitriding would be a good option on the gear teeth, depending on the cheese composition Renault used anything from 50-200% increase in fatigue life.
Group buy of synchros anyone? http://www.synchrotech-transmissions.com/Carbon_Synchros.html