Re-built the gearbox using the internals of Bonnie's original four speeder assembled onto the overdrive output shaft. Just need to replace the selectors (detent ball fun coming up) and re-fit OD.
Luckily I've got a spare clutch and bearing to go in too
Too impatient and put it together with the carbon thrust
It only works box is now quiet, no crunching from fourth to third and no jumping out of fourth and reverse Overdrive even works.
I did have bit of a scare as I could only get 3rd an 4th but after a bit of a run everything loosened up and although still a bit tight it all works nicely. I did manage to yank the gear knob off though, it's got the OD switch embedded in it but the wooden knob is split and useless. I've pinched the one of Ted the traveller for now and will fix the switch in another place.
I will get an MGB thrust and fit it at the next opportunity.