I have just gone through this particular loop with my car, as it has just eaten it's original standard box, and came to an entirely new conclusion…
The standard R-33 box is inherently very strong and able to take far more of a hammering than an R-34 box. The reason for this is quite simple - the more gears you have in a box, the less space there is for each gear. This generally means that the thickness of the gears in a five speed box are thicker than those of a six speed and therefore stronger. There is a six speed conversion available for the R-33 box, but this again runs into the space issue, with the conversion gears being even smaller than the R-34 six speed. There is no doubting that a brand new sequential 6 is the way to go however, you're looking at around £10,000 all up.
The only other option open is to play on the exiting strengths of the R-33 box and expand on them. OSG do a gear kit conversion where they replace the R-33 gears most likely to go pop under full noise, namely 1st, 2nd, 3rd and if you require it, 5th. The replacement gears have a thicker running surface and a correspondingly thinner syncromesh to allow for a generally stronger operation. The ratios are also changed, making 1st, 2nd and 3rd much higher allowing for lower and tighter gearing for the 30Mph - 120Mph arena where you really can use and feel the power.
The reports I have had back from speaking to people who have used this replacement OSG kit are all that the car feels a decidedly faster and is ideally set up for 1/4 mile runs where you can complete the strip still in 3rd (because of the higher ratio) and not have to loose time either snatching 4th just before the line, or over revving out of the power band holding 3rd.
The best part of this kit is still to come - It is roughly a 5th of the cost of a sequential six speed.
Mine will be fitted and ready for battle next week, so I'll post my views on what it's like then.