Originally Posted by BGTV8

The only way to bullet-proof the transmission is to replace it.

You have 2 obvious choices and some not so obvious.

1. QUAIFE QBE69G with syncro on 6th
2. HOLINGER SG3 if you are not boosted and RD6 if you are

The QBE69G and SG3 will be about the same cost and the RD6 a shed-load more.

Other options is
3. TTI 6-speed sequential from New Zealand

These will not break BUT they are race transmissions and straight cut in lower gears and whine like an entitled millenial who has been separated from his mothers nipple too soon.

These options will cost the equivalent of 4-5 OEM replacement transmissions - apart from the RD6 option which will be USD20K+

Simplest solution is replace the OEM trans