For all those who would rather beef up the HZJ105 series R151F gearbox than replace it, here is a list of parts required to replace the partially splined 3rd / 4th Clutch hub with the modified fully splined clutch hub that is currently available for the 78/79 series R151F gearbox.
1 x Modified Clutch Hub 33362-35030M
3 x Shift Keys 33366-35030
2 x Shift Key Springs 33391-25030
1 x Clutch Hub Sleeve 33364-35050
1 x 3rd Gear Synchro 33368-35020
1 x 4th Gear Synchro 33368-35040
1 x 3rd Gear 33034-60030 (ouch - yes this must be replaced however, the input shaft and output shaft don't need replacing)
All available from Don Kyatt or Driveline with a total retail cost around $770 inc GST.
Now for the interesting part, Don Kyatt's Melbourne head office said that plans were underway to manufacture a fully splined version of the 100 series clutch hub but it won't be available for at least a year.
Other items available to beef up the gearbox are the hardened 1st and 5th gear thrust washers, the metal cylindrical spacer and the fully splined 5th gear.
I was also informed that it may be prudent to replace the retaining plate securing the output shaft bearing - 333151-35020 ($52 inc).