@
Subiearz I believe there is an alignment / reverse lockout plate being out of adjustment, too far one way.
quoting another user on adjusting the plate: Put the car in 1st gear, then loosen the 4 reverse lockout plate bolts. Slide the plate to the right until it is barely touching the reverse lockout plate, then tighten the bolts.
Also if you haven't considered it yet, the Perrin/Whiteline "Positive shift kit" should also help with things, maybe not particularly 6th gear but with everything else.