Sorry to read of your gearbox issue Dave. My +8`s engine ran less than smoothly when brought back into use after a lengthy lay-up, at which time my gearbox really rattled when the engine was trying to idle, though seemed fine while under load, I found a split in the diaphragm of one of the carbs and a few other issues that once resolved, removed the rattle from the gearbox..! If nothing as simple as that then perhaps this link might help...

https://www.ashcroft-transmissions.co.uk/manual-gearboxes/r-380.html

I think some +8 folk changed to Toyota gear boxes..

Good luck.