I bought my GT6 in 1978, 30k miles IIRC. Gearbox failed in 1983 at 50k(?) Got a "recovered" one of uncertain heritage, cheap. It's still running fine now at 70k. I've had to regularly replace the linkage bits, twice or more(?) and they need doing again! The block has had two re-bores and re-grinds. I've also had a broken rear spring and boken vertical link In fact in it's 50 years EVERYTHING that can fail has, sometimes twice.
But I'm happy with the gearbox, probably because it has no OD and I'm a poser not a racer!
Doug